67 
Isorerus Gicas. Dekay. Casts Nos. 21 and 22. - 
Head representing a spherical triangle, surface 
punctate, convex, descending from between the eyes 
to the antepior border, which has a narrow raised 
rim; posterior extremity concave and corresponding 
to the articulation of the abdomen. Eyes elevated, 
prominent, sub-pedunculated; cornea oblong, lunated, 
highly polished; abdomen with eight distinct arti- 
culations, the middle lobe double the size of the late- 
ral one: these latter are continuous with the middle 
lobe, have a deep furrow impressed on their upper 
surfaces, which becomes eradually effaced towards 
their narrow free extremities. These lateral lobes 
are rounded at their extremities, and flattened in such 
a manner as to allow each lobe to slide easily under 
the lobe immediately preceding. Tail subtriangular, 
convex, equalling the head in size, with the posterior 
termination rounded. On the centre of its surface, 
when accidentally decorticated, a slight elevation — 
“may be traced, if the specimen be held in a certain 
light, which appears to be a continuation of the mid- 
dle lobe; this extends to within a short distance of 
_the posterior angle of the tail, when it is either en- 
tirely effaced or terminates in an abrupt truncation. 
Another elevation runs parallel to and at a short dis- 
tance from the edge of the tail. These elevations 
are connected by obscure parallel lines, imitating 
the spaces between the laterallobes. When the tailis 
fractured on the borders, a semilunar depression is 
