DECAPODA. 55 



marinus, Fab. which., if it applied to the other Macroura, 

 would be decisive; it is, that besides the two venous sinuses 

 of which we have spoken in our general observations upon 

 the order, there is a third, situated in the sternal canal be- 

 tween the two preceding ones and extending from one end of 

 the thorax to the other. This curious arrangement, accord- 

 ing to them, establishes a connexion between the venous sys- 

 tem of the Macroura, and that of the Stomapoda. 



The Macroura never quit the water, and, with the excep- 

 tion of a small number, are all marine Crustacea. 



In imitation of De Geer and Gronovius, we will arrange them 

 in a single genus (l), that of Astacus, which we divide in the fol- 

 lowing manner: 



Some, by the proportions, figure, and uses of their feet, of which 

 the first, or at least the second pair, are in the form of claws, and by 

 the subcaudal situation of their ova, evidently approach the preceding 

 Crustacea, and approximate still more closely to those commonly 

 known by the names of Craivfish, Lobster, and Shrimp. 



The feet of the others are very slender, and are furnished with an 

 exterior and elongated appendage or branch, which seems to double 

 their number. They are exclusively adapted for natation, and none 

 of them terminates in a forceps. The ova are situated between them, 

 and not under the tail. 



We will subdivide the former into four sections^ the Anomala, the 

 Locusts, the Astacina, and the Carides. 



The latter will compose the fifth and last sections of this family, 

 and of the Decapoda, or that of the Schizopoda. 



In the first, or the Anomala, the two or four last feet are always 

 much smaller than the preceding ones. The under part of the tail is 

 never furnished with more than four pairs of appendages or false 

 feet(2). The lateral fins of the end of the tail, or the pieces which 



(1) The sections which we are about to describe might form so many generic 

 divisions, having for their basis the genera of Fabricius. 



(2) With the exception of the two that are anterior, these appendages in the 

 males are mere rudiments, or are even wanting, a character common to the Gala- 

 theee, Scyllari, and Palinuri. We should also observe that in these three sub- 

 genera the caudal fins are thinner or almost membranous at their posterior extre- 

 mity. In this section, as well as in the Galatheae, the thoracic portion to which the 

 two posterior feet are attached forms a sort of petiole, so that these feet seem to 

 he annexed to the tail. 



