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 
