164 
CRUSTACEA. 
The shell of the last swimmers is m\ich 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, Lam., 
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 | 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 antennee 
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, 1 ; Leach, 
Zool., Miscell. cxlviii ; Portunis vigil, Fab. 
t Hist. Nat. des Crust. Foss., V, 6, 7, 8. 
J Genus Thalamita, Lat. 
