Order Nektaspia nov.

Trilobita without thoracic segments. Cephala and pygidia simple.

Family Naraoidae Walcott.

Cephalon and pygidium large, both shields nearly smooth. Eyes absent.

A single species: _Naraoia compacta_ Walcott, Middle Cambrian, British Columbia.

Subcla.s.s Haplopoda nov.

Crustacea with trilobate form, two pairs of uniramous antennae, no facial sutures, sessile compound eyes present or absent, pygidium and pleural lobes generally reduced, large labrum present, appendages of the trunk biramous.

Order Marrellina nov.

Form trilobite-like, pleural lobes reduced, endobases absent from c.o.xopodites of body, pygidium a small plate.

Family Marrellidae Walcott.

Cephalon with long genal and nuchal spines. Eyes marginal. A single species: _Marrella splendens_ Walcott, Middle Cambrian, British Columbia.

Order Aglaspina Walcott.

Body trilobite-like, with few thoracic segments, and a spine-like telson. Appendages biramous.

Family Aglaspidae Clarke.

Cephalon trilobate, with or without compound eyes, seven or eight segments in the thorax.

Genus _Aglaspis_ Hall.

Compound eyes present, seven segments in thorax. Upper Cambrian, Wisconsin.

Genus _Molaria_ Walcott.

Compound eyes absent, eight segments in thorax. Middle Cambrian, British Columbia.

Genus _Habelia_ Walcott.

Compound eyes absent. Not yet fully described. Middle Cambrian, British Columbia.

Subcla.s.s Xenopoda nov.

Crustacea with more or less eurypterid-like form, one pair of uniramous antennae, biramous appendages on anterior part of trunk, modified endopodites on cephalon.

Order Limulava Walcott.

Cephalon with lateral or marginal eyes and large epistoma. Body with eleven free segments and a telson. Cephalic appendages grouped about the mouth.

Family Sidneyidae Walcott.

Trunk probably with exopodites only, and without appendages on the last two segments. Telson with a pair of lateral swimmerets.

Genus _Sidneyia_ Walcott.

Third cephalic appendage a large compound claw. Gnathobases forming strong jaws. Middle Cambrian, British Columbia.

Genus _Amiella_ Walcott.

Middle Cambrian, British Columbia.

Family Emeraldellidae nov.

Trunk with biramous appendages in anterior part, and appendages on all segments except possibly the spine-like telson.

Genus _Emeraldella_ Walcott.

Cephalic appendages simple spiniferous endopodites. Eyes unknown.

Middle Cambrian, British Columbia.

[Ill.u.s.tration: Fig. 41.--A diagram showing possible lines of descent of the other Arthropoda from the Trilobita. The three recognized orders of the latter are shown separately. The known geological range is indicated in solid black, the hypothetical range and connections stippled. The short branch beside the Opisthoparia represents the range of the Haplopoda. The term Arachnida is used for all arachnids other than Merostomata, merely as a convenient inclusive name for the groups not especially studied.]

Final Summary.

It is generally believed that the Arthropoda const.i.tute a natural, monophyletic group. The data a.s.sembled in the preceding pages indicate that the other Arthropoda were derived directly or indirectly from the Trilobita because:

(1) the trilobites are the oldest known arthropods;

(2) the trilobites of all formations show great variation in the number of trunk segments, but with a tendency for the number to become fixed in each genus;

(3) the trilobites have a constant number of segments in the head;

(4) the position of the mouth is variable, so that either the Crustacea or the Arachnida could be derived from the trilobites;

(5) the trilobite type of appendage is found, in vestigial form at least, throughout the Arthropoda;

(6) the appendages of all other Arthropoda are of forms which could have been derived from those of trilobites;

(7) the appendages of trilobites are the simplest known among the Arthropoda;

(8) the trilobites show practically all known kinds of sessile arthropodan eyes, simple, compound, and aggregate;

(9) the apparent specializations of trilobites, large pleural lobes and pygidia, are primitive, and both suffer reduction within the group.

The ancestor of the trilobite is believed to have been a soft-bodied, free-swimming, flat, blind or nearly blind animal of few segments, because: