A person who is acquainted only with _Cryptolithus_ preserved in shale, or with figures, usually has a very erroneous idea of the fringe It is not a flat border spread out around the front of the head, but stands at an angle about 45 in uncrushed specimens of most species. When viewed from the lower side, there is a single outer, concentric row of the cup-shaped depressions, bounded within by a prominent girder. This row is in an approximately horizontal plane, while the remainder of the doublure of the fringe rises steeply into the hollow of the cephalon. Since the front of the hypostoma is attached to this doublure, it stands high up within the vault and under the glabella. Two specimens, Nos. 231 and 233, show something of the hypostoma, and they are the only ones known of any American trinucleid. That of specimen 233, the better preserved, is very small, straight across the front, and oval behind. It seems that it is abnormally small in this specimen and I should not be surprised if in other specimens it should be found to be larger.

In the Bohemian _Trinucleoides reussi_ (Barrande), the oldest of the trinucleids, the hypostoma is very commonly present, and is of the proper size to just cover the cavity of the glabella, seen from the lower side, and has, toward the anterior end, side flaps which reach out under the prominent glabellar lobes. This large size of the hypostoma would cause the antennules to be attached outside the dorsal furrows, and the position in which they are attached in the American species of _Cryptolithus_ may be explained as an inherited one, since with the small hypostoma they might have been within the glabella, as in _Triarthrus_.

The antennules are seen in three specimens, and in all cases are directed backward. The particular course in which they are drawn in the restoration is purely arbitrary. The second pair of cephalic appendages are represented as directed downward and forward, since in one or two specimens fragments of forward-pointing endopodites were seen near the front of the cephalon, and because in other trilobites the second pair of appendages is always directed forward. The remaining three pairs have a more solid basis in observed fact, for the two or three specimens retaining fragmentary remains of them indicate that they turn backward like those on the thorax, and that the individual segments are longer and more nearly parallel-sided than those of the more posterior appendages. The gnathites of all the cephalic appendages are admittedly purely hypothetical. None of the specimens shows them. As drawn, they are singularly inefficient as jaws, but if, as is suggested by the casts of the intestines of trinucleids found in Bohemia, these trilobites were mud-feeders, inefficient mouth-parts would be quite in order.

[Ill.u.s.tration: Fig. 20. _Cryptolithus tessellatus_ Green. A restoration of the appendages drawn by Doctor Elvira Wood from the original specimens and from the photographs made by Professor Beecher.

9.]

The appendages of the thorax and pygidium can fortunately be taken quite directly from the photographs of the dorsal and ventral sides of well preserved specimens. There is of course a question as to the number and the exact form of those on the pygidium, but I think the present restoration is fairly well justified by the specimens. As would be expected from the narrow axial lobe, the gnathobases of the c.o.xopodites are short and small.

Summary on the Ventral Anatomy of Trilobites.

COMPARISON OF APPENDAGES OF DIFFERENT GENERA.

Since the appendages of _Triarthrus_, _Cryptolithus_, _Neolenus_, _Calymene_, and _Ceraurus_ are now known with some degree of completeness, those of _Isotelus_ somewhat less fully, and something at least of those of _Ptychoparia_, _Kootenia_, and _Acidaspis_, these forms being representatives of all three orders and of seven different families of trilobites, it is of some interest to compare the h.o.m.ologous organs of each.

All in which the various appendages are preserved prove to have a pair of antennules, four pairs of biramous limbs on the cephalon, as many pairs of biramous limbs as there are segments in the thorax, and a variable number of pairs on the pygidium, with, in the case of _Neolenus_ alone, a pair of tactile organs at the posterior end. Each limb, whether of cephalon, thorax, or pygidium, consists of a c.o.xopodite, which is attached on its dorsal side to the ventral integument and supported by an appendifer, an exopodite, and an endopodite. The exopodite is setiferous, and the shaft is of variable form, consisting of one, two, or numerous segments. The endopodite always has six segments, the distal one armed with short movable spines.

_c.o.xopodite._

The c.o.xopodite does not correspond to the protopodite of higher Crustacea, the basipodite remaining as a separate ent.i.ty. The inner end of the c.o.xopodite is prolonged into a flattened or cylindrical process, which on the cephalon is more or less modified to a.s.sist in feeding, and so becomes a gnathobase or gnathite. The inner ends of the c.o.xopodites of the thorax and pygidium are also prolonged in a similar fashion, but are generally somewhat less modified. These organs also undoubtedly a.s.sisted in carrying food forward to the mouth, but since they probably had other functions as well, I prefer to give them the more non-committal name of endobases.

In _Triarthrus_ and _Neolenus_ the endobases are flattened and taper somewhat toward the inward end. In _Isotelus_, _Calymene_ and _Ceraurus_, they appear to have been cylindrical. In other genera they are not yet well known. In all cases, particularly about the mouth, they appear to have been directed somewhat backward from the point of attachment. As it is supposed that these organs moved freely forward and backward, the position in which they occur in the best preserved fossils should indicate something of their natural position when muscles were relaxed.

_Cephalon._

_Antennules._--Antennules are known in _Triarthrus_, _Cryptolithus_, _Neolenus_, and _Ptychoparia_. In all they are long, slender, and composed of numerous segments, which are spiniferous in _Neolenus_, and very probably so in the other genera.

In _Triarthrus_, _Neolenus_, and _Ptychoparia_ they project ahead of the cephalon, emerging quite close together under the front of the glabella, one on either side of the median line. In _Cryptolithus_ they turn backward beneath the body, but since only three or four specimens are known which retain them, it is possible that other specimens would show that these organs were capable of being turned forward as well as backward. The proximal ends of the antennules being ball-like, it is probable, as Doctor Faxon has suggested to me, that these "feelers" had considerable freedom of motion. The antennules of _Triarthrus_ are apparently somewhat less flexible than those of the other genera, and have a double curvature that is seen among the others only in Ptychoparia. The proximal end of an antennule in _Triarthrus_ is a short cylindrical shaft, apparently articulating in a sort of ball-and-socket joint. The proximal end in the other genera is still unknown. The points of attachment in _Triarthrus_ seem to be under the inner part of the second pair of glabellar furrows. In _Cryptolithus_ they appear to be beside the anterior lobe of the glabella under what have long been known as the antennal pits. In the other genera the location is not definitely known, but in _Neolenus_ it seems to be under the dorsal furrows near the anterior end of the glabella. Viewed from the under side, the point of attachment is probably always beside the middle or anterior part of the hypostoma, just behind the side wings.

_Paired biramous appendages._--Behind the antennules all the appendages except those on the a.n.a.l segment are biramous, consisting of a c.o.xopodite with an inward-directed endobase and an outward-directed pair of branches, the exopodite above, and the six-jointed endopodite beneath. The basipodite really bears the exopodite, but the latter also touches the c.o.xopodite. This structure has been seen in _Triarthrus_, _Cryptolithus_, _Neolenus_, _Kootenia_, _Calymene_, _Ceraurus_, and _Ptychoparia_. In _Triarthrus_, _Neolenus_, _Acidaspis_, _Ptyclioparia_, and Kootenia, the appendages extend beyond the margins of the dorsal test. In _Cryptolithus_ and _Isotelus_ none (other than antennules) does so. In _Isotelus_ and _Acidaspis_ only the endopodites have been seen. In _Triarthrus_, _Calymene_, _Ceraurus_, and _Neolenus_ there are four pairs of appendages behind the antennules. The other genera probably had the same number, but the full structure of the under part of their cephala is not known. In _Triarthrus_ the endopodites of the cephalon are slender, the individual segments parallel-sided, the inner ones flattened, the outer ones cylindrical in section. They project slightly beyond the edge of the cephalon when fully extended, and each terminates in three small spines. In _Cryptolithus_ the endopodites of the cephalon are longer than those of the thorax, but with the possible exception of the first pair, are bent backward at the carpopodite, and do not ordinarily project beyond the brim of the test. In _Neolenus_ the endopodites of the cephalon are rather thick and wide, but are long, project forward, and extend beyond the brim.

The individual segments are flattened, probably compressed oval in section. The terminal segment of each is furnished with three strong spines at its distal end. In _Calymene_ and _Ceraurus_ the endopodites appear to consist of slender segments which are oval or circular in section. In _Calymene_ Walcott believed the three distal segments of the last endopodites of the head to be greatly enlarged, giving these appendages a paddle-like form similar to some of the appendages of eurypterids. The evidence for this does not seem to me to be good. The cephalic endopodites of _Isotelus_ are entirely similar to those of the thorax, and are rather short, consisting of a series of short cylindrical segments which do not taper greatly toward the distal end.

The endopodites of the cephalon of _Acidaspis_, _Kootenia_, and _Ptychoparia_ are still unknown.

The exopodites of the cephalon seem in all known cases (_Triarthrus_, _Cryptolithus_, _Neolenus_, and Ceraurus) to be like those of the thorax. They point more directly forward in most cases, project beyond the margin of the head normally only in Triarthrus, and usually occupy the region under the cheeks (fixed and free).

The endobases of the c.o.xopodites of the appendages of the cephalon probably in all cases function as mouth-parts (gnathites), and are especially modified for this purpose in Triarthrus, being flattened, shoe-shaped in outline, and so arranged that they work over one another in a shearing fashion. While the more anterior of the c.o.xopodites are attached in front of the posterior tip of the hypostoma, the gnathites of Triarthrus bend backward so that all are behind the hypostoma. In _Calymene_ and _Ceraurus_, two or three pairs of the gnathites are back of the hypostoma, and one or more pairs may be beside or under the hypostoma. In these genera the mouth is probably in front of the tip of the upper lip. In _Isotelus_, the mouth seems to have been situated in the notch between the two branches of the hypostoma, and the gnathites of two or three pairs of the appendages probably worked under its forks. Since the length of the hypostoma differs in the various species of _Isotelus_, there would be a variable number of gnathites projecting under its forks, according to the species. In this genus the gnathites are of the same long form, cylindrical in cross-section, as the endobases of the thoracic segments, but each is bowed back considerably from the point of attachment.

The gnathites of _Neolenus_ are like the endobases of the thorax, but broader. The great length of the hypostoma makes it probable that the mouth was far back and that some of the gnathites were in front of it.

The gnathites of _Cryptolithus_ are unknown. Professor Beecher in his drawing shows some fragments with toothed ends near the hypostoma, and it may be that they are inner ends of gnathites, but I see nothing to substantiate such an interpretation. If, as some suppose, _Cryptolithus_ was a mud feeder, the gnathites were probably poorly developed. Of the gnathites of _Kootenia_, _Ptychoparia_, and _Acidaspis_ also nothing is known.

_Thorax._

In each genus there is a pair of appendages for each segment of the thorax. When the axial lobe is narrow, the endobases of the c.o.xopodites are small and short (_Cryptolithus_, _Ceraurus_, _Calymene_). When the axial lobe is wide, the endobases are long and stout (_Isotelus_, _Triarthrus_). The exopodites always lie above and in front of the corresponding endopodites. In Triarthrus the two branches are of practically equal length. In _Cryptolithus_ the exopodites are much the longer. In _Neolenus_, _Calymene_, _Ceraurus_, _Kootenia_, and _Ptychoparia_, the exopodites are shorter than the endopodites.

The exopodites in Triarthrus consist of a proximal shaft, succeeded by numerous short segments, and ending distally in a long, grooved, somewhat spatula-shaped segment. Along the anterior margin of the shaft there are many small spines. Along the posterior margin there are numerous flattened setae, which all lie in one plane and which seem to be more or less united to one another like the barbs of a feather.

The setae are short, not much longer than the width of one of the thoracic segments, and point backward and outward. In _Cryptolithus_ the shaft does not seem to be made up of small segments, and is narrow, with a decided backward curve. The setae are considerably longer and much more flattened than in Triarthrus. In _Calymene_ the state of preservation does not allow a very full knowledge of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate setae. The coiled branches of the exopodites as described by Walcott seem to me to be only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the view of Beecher. In _Ceraurus_ the exopodite seems to have been somewhat paddle-shaped, expanded at the distal end, and to have had rather thick, blade-like setae.

The exopodite of _Neolenus_ is decidedly leaf-like, and reminds one somewhat of the exites of some of the phyllopods. The shaft is a broad unsegmented blade. The setae are slender, delicate, flattened, and a little longer than the width of the shaft. The exopodites of this genus point forward all along the body. In _Kootenia_ the exopodites are like those of _Neolenus_, but with a narrower shaft.

The exopodites of _Ptychoparia_ appear to be very much like those of Triarthrus, but the shaft is probably not segmented.

The endopodites of the thorax of _Triarthrus_, _Cryptolithus_, and _Acidaspis_ show progressive modification from front to back in the broadening of the individual segments and the a.s.sumption by them of a triangular form. Not only do the individual segments become more triangular from front to back, but more of the segments of each endopodite become triangular. This modification has so far been seen in these three genera only. The individual segments, except the distal ones, seem to be flattened in all these genera. The distal end of the terminal segment of each endopodite of _Triarthrus_ bears three small movable spines, and each of the segments usually bears three or more spines, located in sockets along the dorsal surface and at the anterior distal angle of each segment. The endopodite of _Cryptolithus_ is bent backward at the carpopodite and this segment is always thickened. At the distal end of the dactylopodite there is a tuft of spines, the triangular segments have tufts of spines on their posterior corners, and there are groups of spines also in the neighborhood of the articulations.

The endopodites of _Ceraurus_, _Calymene_, and _Isotelus_ are all relatively slender, the segments are parallel-sided, and there seems to be no particular modification from front to back of the thorax. The endopodites of _Isotelus_ are short, the entire six segments of one being but little longer than the c.o.xopodite of the same appendage. The segments of the endopodites of _Neolenus_ are mostly short and wide, and at the distal end of the terminal segment there are three stout spines. In _Kootenia_ the endopodites are long and very slender. The endopodites of Ptychoparia are too poorly preserved to show details, and those of the thorax of _Acidaspis_ likewise reveal little structure, but they seem to have the triangular modification, and to turn back somewhat sharply at about the position of the carpopodite.

_Pygidium._

Beecher showed that in _Triarthrus_ there was a pair of appendages on the pygidium for every segment of which it is composed except the last or a.n.a.l segment (protopygidium). Walcott has since shown that in _Neolenus_ this segment bears a pair of cerci, and Beecher's drawings show that in his later studies he recognized a spinous plate, the possible bearer of cerci, on the a.n.a.l segment of _Triarthrus_. The appendages of the a.n.a.l segment have not yet been seen on other species of trilobites.

The appendages of the pygidium do not show any special modifications, but seem in all cases to be similar to those of the posterior part of the thorax. In _Cryptolithus_ all the pygidial appendages are short and remain beneath the cover of the dorsal test, while in _Triarthrus_ and _Neolenus_ they extend behind it.

In the latter genus the endopodites of the pygidial appendages appear to be practically identical in form with those of the thorax, the individual segments being perhaps a little more nearly square in outline. Like those of the thorax, the segments of the pygidial endopodites bear numerous short spines. The caudal cerci are richly segmented, slightly flexible, spinous tactile organs. They are symmetrically placed, nearly straight when in their natural position, and make an angle of about 75 with one another. They appear to be attached to a narrow rim-like plate which seems to fit in just ahead of the doublure of the pygidium, or perhaps over it.

In _Ceraurus_, _Calymene_, and _Isotelus_, the endopodites of the pygidium are similar to those of the thorax, but seemingly more slender, with less well developed c.o.xopodites, and with, in the last-named genus, slender cylindrical segments. Exopodites are not known on the pygidia of any of these genera, but since they are present and like those of the thorax in _Triarthrus_, _Cryptolithus_, _Neolenus_, and _Ptychoparia_, there is little reason to think that they were absent in _Ceraurus_ or _Calymene_, though there is some question about _Isotelus_.

The limbs are largest and longest on the anterior part of the thorax of a trilobite, and diminish regularly in length and strength to the posterior end of the pygidium. This regular gradation shows, as Beecher was the first to point out, that the growing point of the trilobites is, as in other arthropods, in front of the a.n.a.l segment.

New _free_ segments are introduced into the thorax at the anterior end of the pygidium, and this has led to some confusion between the growing point and the place of introduction of free segments.

If a new segment were introduced at a moult in front of the pygidium, that segment would probably have less fully developed appendages than those adjacent to it, and so make a break in the regular succession.

The condition of the appendages corroborates the evidence derived from the ontogeny of the pygidium, and proves that the new segments are introduced at the same growing point as in other Arthropoda.

_Caudal Rami._

Bernard, who believed that the Crustacea had been derived through an _Apus_-like ancestor (1892, pp. 20, 85, 274), pointed out that four or less than four a.n.a.l cirri were to be expected. Two well developed cirri and two rudimentary ones are present in _Apus_, and they are also to be found in other phyllopods and some isopods. It is, however, characteristic of the Crustacea as a whole to lack appendages on the a.n.a.l segment. Caudal cirri (cerci) are much more freely developed in the hexapods than in the Crustacea, particularly in the more primitive orders, Palaeodictyoptera, Apterygota, Archiptera, and Neuroptera. They are supposed, in this case, to be modified limbs, and therefore not h.o.m.ologous with the bristles on the a.n.a.l segment of an annelid. Doctor W. M. Wheeler of the Bussey Inst.i.tution has kindly allowed me to quote the following excerpt from a letter to me, as expressing the opinion of one who has made an extensive study of the embryology of insects:

I would say that I have no doubt that the cerci of insects are directly inherited from the insect ancestors. They are always highly developed in the lower insects, and only absent or vestigial in a few of the most highly specialized orders such as the Hemiptera, Diptera, and Hymenoptera. I have further no doubt concerning their being originally ambulatory in function. They are certainly not developed independently in insects. Embryologically they arise precisely like the legs, and each cercus contains a diverticulum of the mesoblastic somite precisely as is the case with the ambulatory legs and mouth parts.

The "pygidial antennae" seem to be as fully developed in _Neolenus_ as in any of the other arthropods, and may suggest a common ancestry of the phyllopods, isopods, and hexapods, in the trilobites. They were doubtless tactile organs, and while the evidence is chiefly negative, it would seem that they proved useless, and were lost early in the phylogeny of this group. Possibly the use of the pygidium as a swimming organ proved destructive to them.

h.o.m.oLOGY OF THE CEPHALIC APPENDAGES WITH THOSE OF OTHER CRUSTACEA.

The head of the typical crustacean bears five pairs of appendages, namely, the antennules, antennas, mandibles, and first and second maxillae, or, as they are more properly called, the maxillulae and maxillae.

As Beecher has pointed out, the "antennae" of the trilobites, on account of their pre-oral position and invariably uniramous character, are quite certainly to be correlated with the antennules.

The second pair of appendages, the first pair of biramous ones, Beecher h.o.m.ologized with the antennae of other crustaceans, and that h.o.m.ology has been generally accepted, though Kingsley (1897) suggested that it was possible that no representatives of the true antennae were present.

In preparing the restorations in the present study, the greatest difficulty has been to adjust the organs about the mouth. In _Triarthrus_, numerous specimens show that without question there are four pairs of gnathites back of the hypostoma, and that all four belong to the cephalon. In forms with a long hypostoma, however, there was no room on the cephalon for the attachment of four pairs of gnathites, neither were there enough appendifers to supply the requisite fulcra. At first I supposed I had solved the difficulty by a.s.suming the mouth to be in front of the posterior tip of the hypostoma, as it really is in Ceraurus and _Calymene_, and allowing the gnathites to play under the hypostoma as Walcott (1912) has shown that they do in _Marrella_. Finally, when I came to study in greater detail the slices of _Calymene_ and _Ceraurus_, they seemed to show that the anterior one or two pairs of appendages became degenerate and under-developed. This was probably a specialization due to the great development of the hypostoma in trilobites, that organ being much more prominent in this than in any other group. As the hypostoma lengthened to accommodate the increasing size of sub-glabellar organs (stomach, heart, etc.), the mouth migrated backward, leaving the anterior appendages ahead of it, with their gnathobases, at least, functionless. That such migration has taken place, even in Triarthrus, is shown by the fact that the points of articulation of the first biramous appendages are pre-oral, and it is more obviously true of _Ceraurus_. Correlated with the weakening of the appendages on the lower surface is the loss of glabellar furrows on the upper surface.

The glabellar furrows mark lines of infolding of the test to form the appendifers and other rugosities for the attachment of tendons and muscles. It is conceivable that this migration backward of the mouth began very early in the history of the race, and that even before Cambrian times, the antennae, probably originally biramous appendages like those on the remainder of the body, had dwindled away and become lost. If this is the case, then the first pair of biramous appendages of _Triarthrus_ would be mandibles, the second pair maxillulae, and the third pair maxillae.

There remain the last pair of cephalic appendages, and they bring up the whole head problem of the trilobites. Beecher has stated (1897 A, p. 96) his conviction that the head of the trilobite is made up of five segments, representing the third, fourth, fifth, sixth, and seventh neuromeres of the theoretical crustacean. As a matter of fact, he really made up the head of seven segments, since he stated that the first neuromere was represented by the hypostoma and the second by the epistoma and free cheeks.

Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight segments, as he saw five segments in the glabella of certain trilobites. In his table (p. 165) he has listed the segments with their appendages as follows: 1. Acron, with hypostoma; 2, rostrum (epistoma), with free cheeks; 3, first frontal lobe, with (?) antennules; 4, second frontal lobe, with antennae; 5, mandibles; 6, first, or pre-maxillae; 7, second maxillae; 8, occipital segment with maxillipeds.