CRUSTACEA. 167 



The nuchal furrow takes its origin upon the dorsal line just in front of the 

 centre and passes obliquely forward to the antero-lateral margin, cutting off 

 a sub-triangular area on each valve. This double area constitutes the 



Cephalic region, which, upon the anterior portion of each valve, bears a 

 broad, low node, a small elevated tubercle near the hinge, and a small node 

 near the nuchal furrow, distant from the hinge. The last mentioned node 

 upon each valve is usually slightly depressed at the summit, and occupies 

 the position of the optic spot or eye. 



Thoracic region large, anteriorly nodose, smooth and evenly convex behind. 

 Just behind the nuchal furrow and close upon the hinge-line, there is a 

 transversely elongate node, with its summit directed dorsally; below it an 

 obliquely elongate node without apex. These two nodes are separated from 

 the broad and evenly rounded postero-dorsal area by a conspicuous furrow. 

 On the ventral area is a sharply elevated and conspicuous carina, beginning- 

 near the antero-lateral margin, and rounding in a broadly sigmoid curve to 

 the posterior extremity of the carapace. The anterior curve of this ridge is 

 dorsal, the posterior ventral, and the ridge itself is more strongly excavate 

 upon its upper or dorsal slope. Under favorable preservation the area 

 between the carina and the ventral margin shows a series of fine, undu- 

 lating, elevated lines, radiating from the ventral edge of the carina. 



Surface of the carapace over the cephalic region covered with low, faint 

 pustules, which become more conspicuous upon the thoracic region. Upon 

 the ventral face of the carina, and at the ventral margin, these pustules 

 become elongated and much stronger. In specimens from the sandy shales 

 of the central counties these markings appear as punctae. 



Mandibles. Two examples have been observed which retain these organs in 

 their approximately normal position beneath the carapace. Of these one 

 is a small individual measuring about 60 mm. in entire length, which has 

 been described and figured by Mr. Beecher {loc. cit. fig. 16); the other a very 

 large individual, measuring 185 mm. in entire length, in which the mandibles 

 have been somewhat flattened but not much dislocated. These bodies are 



