CRUSTACEA. 185 



near the base of the cephalic region, a moderately large, semi-circular 

 node is sometimes seen, which is more distinct in specimens where the 

 carapace has been somewhat compressed. 



The Surface is ornamented by elevated anastomosing lines, which are 

 absent on the thoracic region within the outer carina. The hinge-line and 

 the posterior margin are distinctly crenulated. 



Abdomen. This part has not been found in conjunction with the carapace. A 

 small abdomen, composed of two somites and bearing a portion of a cylin- 

 drical telson, which was found in the shales with these carapaces, was refer- 

 red, in the original description, to this species, and it either belongs to this 

 form or to some species as yet undescribed. These somites are sub-cylin- 

 drical, the first short, wider than long, and the second twice as long as wide, 

 depressed at the anterior margins, slightly elevated and overlapping at the 

 posterior margins, and ornamented with elevated lines similar to those upon 

 the carapace. This specimen of the abdomen is small, and must have be- 

 longed to an individual smaller than indicated by any carapace yet observed. 



Dimensions. The best preserved of the type specimens measures 89 mm. 

 in length and 27 mm in width. The two somites of the abdomen have a 

 length of 11 mm. of which the first measures 4 mm. and the second 7 mm. 



Distribution. Chemung group. In the shales associated with Echinocaris 

 socialis and Elymocaris siliqua. Warren, Warren county, Pennsylvania. 



Tropidocaris interrupta. 



PLATE XXXI, FI(i. 13 



Tropidocaris interrupta. Beecher. Ceratiocaiidae from the Chemung and Waverly groups, etc. Second Geol. 



Surv. Penna., vol. PPP, p. IS, pi. ii, fig. 6. 1S84. 

 Tropidocaris interrupta, Etueridge, Woodward and Jones. Third Rept. Committee on Fossil Phyllopoda 



of the Palaeozoic Rocks, p. 35. 1885. 



Cephalothorax sub-oval in outline, reentrantly truncate behind. The right 

 valve is the only part yet observed, and this shows the hinge-line to be 

 straight, shorter than the greatest length of the carapace ; the anterior 

 extremity acute, the margin rapidly rounding thence to the centre of the 



