THE GENERAL NAME, GENUS. 247 



the free end of the latter and being nearly parallel with it. 

 In the first appendage (C), the anterior rolled edge (a) 

 more closely embraces the posterior (6), and the groove 

 is more completely converted into a tube. 



It will be observed that the differences between the 

 English and the Californian crayfishes amount to ex- 

 ceedingty little ; but, on the assumption that these differ- 

 ences are constant, and that no transitional forms between 

 the English and the Californian crayfishes are to be 

 let with, the individuals which present the characteristic 

 )eculiarities of the latter are said to form a distinct species, 

 [stacus nigrescens ; and the definition of that species is, 

 Ike that of the English species, a morphological abstrac- 

 tion, embod3ing an account of the plan of that species, 

 so far as it is distinct from that of other crayfishes. 



We shall see by and by that there are sundry other 



dnds of crayfishes, w^hich differ no more from the English 



[or the Californian kinds, than these do from one an- 



[other ; and, therefore, they are all grouped as species of 



the one genus, Astacus, 



If, leaving California, we cross the Rocky Mountains 



ind enter the eastern States of the North American 



[Union, many sorts of crayfishes, which would at once be 



[recognised as such by any English visitor, will be found 



[to be abundant. But on careful examination it will be 



discovered that all of these differ, both from the English 



crayfish, and from Astacus nigrescens, to a much greater 



