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 39 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 Elijmocaris siliqua. Warren, Warren county, Pennsylvania. 
Tropidocaris interrupta. 
PLATE XXXI, FIG. 13. 
Tropidocaris interrupta, Beecher. Ceratiocaridae from the Chemung and Waverly gi-oups, etc. Second Geol. 
Surv. Penna., vol. PPP, p. 18, pi. ii, fig. 6. 1884. 
Tropidocaris interrupta, Etheridge, Woodward and Jones. Third Kept. Committee on Fossil Phyllopoda 
of the PaliEOzoic 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 
