67 



ISOTELUS GIGAS. Defray. Casts Nos. 21 and 22. 



Head representing a spherical triangle, surface 

 punctate, convex, descending from between the eyes 

 to the anterior 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 gradually 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 lateral lobes. When the tail is 

 fractured on the borders, a semilunar depression is 



