Observations on Tr Mobiles. 177 



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 round- 

 ed. On the centre of its surface, when accidentally decorti- 

 cated, a slight elevation may be traced, if the specimen be 

 held in a certain light, which appears to be a continuation of 

 the middle lobe ; this extends to within a short distance of 

 the posterior angle of the tail, when it is either entirely effaced, 

 or terminates in an abrupt truncation. Another elevation 

 runs parallel to, and at a short distance from the edge of the 

 tail. These elevations are connected by obscure parallel 

 iines, imitating the spaces between the lateral lobes. When 

 the tail is fractured on the borders, a semi-lunar depression is 

 visible, exhibiting concentric stria*. The whole surface of the 

 animal has a jet black polish. 



Total length, 6 — 12 English inches. 



Cabinet of the Lyceum. 



This animal, it will be seen, varies much in size ; but the 

 following dimensions are taken from the individual represented 

 in the plate, where it is of course much diminished. 



* The word tail, is used in the common acceptation of the term, although 

 as it often contains a portion of the alimentary canal, or the branchia, or 

 both, it might be termed the post-abdomen. The analogy between the parts 

 of Crustacea and the vertebrated animals, is, however, very remote; still we 

 should avoid coining a new word, unless it becomes necessary to express a 

 new idea. 



23 



