PLATE 6. 

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



Fig. i. Triarthrus bccki. Specimen 203. A well preserved small individual, showing 

 the appendages of the right side of the thorax, x 11.46. 



Fig. 2. Triarthrus bccki. Specimen 209. A well preserved individual, showing the 

 antennules and some appendages of thorax and pygidium. For detail of the pygidium, see 

 pi. 5, fig. i. <4. 



Fig. 3. Triarthrus bccki. Specimen 218. Ventral side of the pygidium and greater 

 part of the thorax of an individual of medium size. Note especially the relation of exopo- 

 dites to endopodites of the'last two thoracic segments. A drawing of these appendages is 

 shown on text fig. 43. x 4,3. 



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



n 



'> : 



II 



Bassler. 



Fig. 6. Acidaspis trentoncnsis Walcott. Both the specimen, No. 245, and the photo- 

 graph are poor, but show that in this genus the endopodites are like those of Triarthrus. 

 x8.5. 



Fig. 7. Cryptolitlius tessellatus Green. Specimen 234. This specimen shows well the 

 backward directed antennules and also the outer segments of some of the cephalic endopo- 

 dites. XII. 



