522 CRUSTACEA. 



tions it in his Faun. Japon., p. 160, stating also, that these species 

 are thereby related to the Thalassinidea. Erichson, on examination, 

 found that the same was true of the A. pellucidus, A. carolinus, A. 

 cubensis, and A. mexicanus, other American species. This law has its 

 exceptions, since the author has found an Oregon species, A. lenius- 

 culus, D., in which the fifth pair of legs has its pair of branchiae, as in 

 the European species. 



Among the distinctions subdividing the genus Astacus, that of the 

 presence or absence of prehensile appendages to the first abdominal seg- 

 ment in males, fitted for use in coition, appears to be of the first impor- 

 tance. These appendages are long in the European and American Astaci, 

 and those of the second pair are also modified for the same end, so as 

 to differ from those of the third and following pairs. But in the 

 Madagascar and most Australian species, according to Erichson, these 

 appendages are wanting, and the second pair are like the following. 

 In the former, the caudal segment is divided transversely, and in the 

 latter, it is not at all divided, or imperfectly so. But the texture of 

 the caudal segment, whether calcareous or not to its tip, cannot be of 

 much value in classification, for it varies in the same species with 

 age, and must, therefore, be somewhat dependent on the size of the 

 species. The presence of a branchia to the posterior pair of legs may 

 prove to be a characteristic of importance, requiring a subdivision 

 accordingly ; but of this we doubt. In the American species without 

 this branchia,, which the author has examined, the medial postero- 

 dorsal region of the carapax is narrow linear, while in the European 

 species, and that from Oregon, having the full number of branchiae, 

 this region is quite broad. But we cannot say how far this is gene- 

 rally true. 



For the reasons stated, we accept of Astacoides as a distinct genus, 

 separated from Astacus by the absence of appendages from the first 

 segment of the abdomen ; and we unite with it, Cheraps and Engasus 

 of Erichson. The occurrence of the Engaai in holes in moist earth, is 

 not peculiar to that group, for the same habit has been observed by 

 Prof. S. F. Baird in an American species. Cheraps may perhaps 

 be retained as a subgenus under Astacoides, on account of the absence 

 of the posterior branchiae ; and on the same ground, and no other of 

 importance, Cambarus may be received as a subgenus under Astacus. 



Another genus has been added to the Astacidse by Adam White, 

 called Paranephrops. It has the basal scale of the outer antennae 



