164 CRUSTACEA. 



The shell of the last swimmers is much wider before than behind, 

 forming either the segment of a circle narrowed towards the tail 

 and truncated, or a trapezium, or is almost in the shape of a heart. 

 Its greatest transverse diameter generally surpasses the opposite one. 

 There are but five segments in the tail of the males, instead of the 

 seven found in that of the females, the number usually peculiar to 

 the tail of the Decapoda ; the third and the two following ones are 

 confounded or form but one ; frequently, however, traces of them are 

 discovered, at least on the sides. 



We will first separate those whose eyes are supported by very long 

 and slender pedicles, arising from the middle of the anterior margin 

 of the shell, extending to its lateral angles, and received into a 

 groove run under the edge. Such is the 



PODOPHTHALMUS, La?7l., 



Where the shell forms a transverse trapezium, wider and straight 

 before with a long spiniform tooth behind the ocular cavities. The 

 claws are elongated, spiny, and similar to those of most of the species 

 of the genus LUPA, Leach. 



The only living species known * inhabits the coasts of the Isle 

 of France, and those of the neighbouring seas. 



The valuable cabinet of one of the most learned fossil con- 

 chylidogists of Europe, contains au internal cast of a fossil 

 Podophthalmus, to which M. Desmarest has affixed the name 

 of its possessor, M. de France f . 



The ocular pedicles of the other Crustacea, belonging to this sec- 

 tion, are short, occupy but a very small portion of the transverse 

 diameter of the shell, are placed in oval cavities, and resemble, gene- 

 rally, those of the ordinary Crabs with which these swimmers are 

 almost insensibly connected. They may all be united in one single 

 subgenus, that of 



PORTUNUS, Fab. 



Certain species J peculiar to the Indian Ocean, such as the Admete, 

 Herbst., LVII, 1, are distinguished from all the following ones by 

 their shell, which is of a transversely quadrilateral form, narrowed 

 posteriorly, and whose ocular cavities occupy its anterior lateral 

 angles ; the eyes are thus separated by an interval almost equal to 

 the greatest width of the shell. The insertion of the lateral antennae 

 is at a considerable distance from these cavities. 



Other species, whose shell forms the segment of a circle, poste- 

 riorly truncated and widest in the middle are remarkable for the 

 length of their claws, which is at least double that of the shell. 

 Each side presents nine teeth, the posterior largest and spiniform. 

 The tail of the males is frequently very different from that of the 

 females. 



* Podophthalmus spinosus, Latr., Gener. Crust, et Insect., I, 1, and II, l ; Leach, 

 Zool., Miscell. cxlviii ; Portunis vigil, Fab. 

 f Hist. Nat. des Crust. Foss., V, 6, 7, 8. 

 J Genus THALAMITA, Lat. 



