PLATE 5. 

 Photographs of Triarthrus bccki, made by C. E. Beecher. 



Fig. i. Specimen 209. Photograph of the pygidium shown in pi. 6, fig. 2. This 

 specimen shows especially well the way in which the exopodites of the pygidium decrease 

 in length backward, x 11.5. 



Fig. 2. Specimen 229. The under side of the posterior end of a medium-sized speci- 

 men, 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, x 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, x 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 shell. Whether the curved bodies have 

 anything to do with the trilobite is not known, x 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, 

 x ii. 



Fig. 6. Specimen 202. Posterior part of the thorax and pygidium, showing endopo- 

 dites 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. x 9-75- 



