2.4.

Fig. 3. Specimen 216. A small entire specimen showing considerable of the detail of the appendages of the cephalon, and some of those of the remainder of the body, 7.4.

Fig. 4. Specimen 201. This figure shows the details of the appendages of the left side and of the pygidium. Note the plate on the median line back of the pygidium, the sockets for spines, and the terminal spines on the anterior endopodites. See also pl. 2, fig. 1 and pl. 4, figs. 1, 2. 7.1.

Fig. 5. Specimen 207. One half of the posterior part of the thorax and pygidium, showing exopodites and endopodites as seen from the dorsal side, 7.6.

Fig. 6. Specimen 214. The exopodites have been turned back nearly parallel to the axis of the sh.e.l.l. Notice particularly the long flattened setae and the spinose spatula-shaped terminal portion of each shaft. See also pl. 1, fig. 2. 7.

PLATE III.

HELIOTYPE CO. BOSTON

PLATE 4.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 201. Another photograph, similar to fig. 4, pl. 3, but showing more clearly some details of spines on the endopodites.

12.66.

Fig. 2. Specimen 201. Three appendages on the right side of the thorax. See also pl. 2, fig. 1 and pl. 3, fig. 4. 12.66.

Fig. 3. Specimen 223. A small crushed specimen which nevertheless shows well the appendages of the right side of the thorax, developed from the ventral side. Note c.o.xopodites, exopodites, and endopodites, and that all appendages are moved equally laterally from their original position. 11.4.

Fig. 4. Specimen 219. Another photograph, with different lighting, of the individual shown in pl. 2, fig. 6. This print brings out better the c.o.xopodites and the folds of the ventral membrane. 3.23.

Fig. 5. Specimen 222. This specimen is interesting, because it shows the endopodites in what is probably their natural position, that is, in a plane nearly vertical to the plane of the body, instead of being flattened down, as is usually the case. The appendages under the pygidium are unusually well preserved. 12.

Fig. 6. Specimen 204. Photograph of the entire specimen of which a part is shown in text fig. 42 and pl. 3, fig. 1. 4.5.

PLATE IV.

HELIOTYPE CO. BOSTON

PLATE 5.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 209. Photograph of the pygidium shown in pl. 6, fig.

2. This specimen shows especially well the way in which the exopodites of the pygidium decrease in length backward, 11.5.

Fig. 2. Specimen 229. The under side of the posterior end of a medium-sized specimen, showing the appendages, especially the endopodites. On and among the limbs are scattered numerous minute spheres of pyrite, of the kind usually known as "trilobite eggs." They do not show very well in the photograph, but can be made out much more clearly with a hand lens, 12.

Fig. 3. Specimen 230. A specimen showing the appendages of the posterior part of the thorax and the pygidium. The same individual is also shown in text fig. 44. Note particularly the form of the segments of the endopodites, and the spines on them, 13.

Fig. 4. Specimen 227. The small doubly curved bodies shown in this figure lie under the axial portion of the cephalon and anterior part of the thorax. The specimen still has a very thin coating of matrix between it and the sh.e.l.l. Whether the curved bodies have anything to do with the trilobite is not known, about 12.

Fig. 5. Specimen 221. A small individual which shows well the exopodites of the posterior part of the thorax. Note the spatulate terminations and the spines of the shaft, 11.

Fig. 6. Specimen 202. Posterior part of the thorax and pygidium, showing endopodites and exopodites projecting under the dorsal test.

Note the spiniferous plate on the median line, and the large opening in the anterior portion of it. 9.75

PLATE V.

HELIOTYPE CO. BOSTON

PLATE 6.

All figures except 4 and 5, from photographs by C. E. Beecher.

Fig. 1. _Triarthrus becki_. Specimen 203. A well preserved small individual, showing the appendages of the right side of the thorax.

11.46.

Fig. 2. _Triarthrus becki_. Specimen 209. A well preserved individual, showing the antennules and some appendages of thorax and pygidium. For detail of the pygidium, see pl. 5, fig. 1. 4.

Fig. 3. _Triarthrus becki_. Specimen 218. Ventral side of the pygidium and greater part of the thorax of an individual of medium size. Note especially the relation of exopodites to endopodites of the last two thoracic segments. A drawing of these appendages is shown on text fig.

43. 4,3.

Figs. 4 and 5. Endopodites, probably from a species of _Calymene_.

These specimens, with several others, are on a small slab of limestone from the Point Pleasant (Trenton) beds opposite Cincinnati, Ohio.

Specimen in the U. S. National Museum. Photographs by R. S. Ba.s.sler.

Fig. 6. _Acidaspis trentonensis_ Walcott. Both the specimen, No. 245, and the photograph are poor, but show that in this genus the endopodites are like those of Triarthrus. 8.5.

Fig. 7. _Cryptolithus tessellatus_ Green. Specimen 234. This specimen shows well the backward directed antennules and also the outer segments of some of the cephalic endopodites. 11.

PLATE VI.

HELIOTYPE CO. BOSTON