DECAPODA. 41 



widely separated on each lateral margin; the posterior largest 

 and spiniform, and situated ahove the origin of the posterior 

 feet. 



The sea coast of the western departments of France produces 

 some other species which belong to the genus Ebalia, Leach(l). 

 All the remaining ones are from India and America. 



Some fossil Leucosis are found in the East Indies. Three 

 species have been described by M. Desmarest, two of which, 

 according to him, are true Leucosiae, Leach, and which are 

 now living in the same countries, and peculiar to them. 

 Our fifth section, that of the Thigona, is composed of those spe- 

 cies whose shell is usually triangular or subovoid, narrowed before 

 into a point or kind of beak, generally uneven and rough, with late- 

 ral eyes. The interval comprised between the antennae and the 

 buccal cavity, is always nearly square, as long, or almost as long as 

 broad. The claws, at least those of the males, are always large and 

 elongated. The following feet are very long in a great number, and 

 sometimes the two last even differ in form from the preceding ones. 

 The third joint of the external foot-jaws is always nearly square or 

 hexagonal, in those at least whose feet are of the ordinary length. 



The apparent number of the caudal segments varies. In both 

 sexes of several it is seven; in others, however, the males at least, 

 it is less. 



Several of these Crustacea are designated by the vulgar appella- 

 tion of Jlraignees tie mer or Sea-spiders. 



Although the species of this tribe are very numerous, but two 

 have as yet been discovered except in a fossil state, one of which at 

 least Maia squinaclo exists at the present day in a living state, and 

 in the same localities(2). 



A first division will comprehend those whose second and follow- 

 ing feet are similar, and which diminish progressively in size. 



From the latter we will form a first group of all those where the 

 tail, either in both sexes, or in the females alone, is composed of 

 seven segments. The third joint of the external foot-jaws is almost 

 always square, and truncated or notched at the superior internal 

 angle. 



Very large claws, particularly so when compared with the other 

 feet, which are extremely short, directed horizontally and perpen- 

 dicularly to the axis of the body as far as the carpus or joint im- 

 mediately preceding the hand, then reflected anteriorly on them- 



(1) Make. Brit., xxv. 



(2) See Desmar., Hist. Nat. des Crust. Foss 



Vol. III. F 



