THE GENEEAL NAME, GENUS. 247 



the free end of the latter and being nearl}^ 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- 

 ceedingly 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 

 met with, the individuals which present the characteristic 

 peculiarities of the latter are said to form a distinct species, 

 Astacus nigrescens ; and the definition of that species is, 

 like that of the English species, a morphological abstrac- 

 tion, embodying 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 

 kinds of crayfishes, which differ no more from the English 

 or the Californian lands, than these do from one an- 



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



' the one genus, Astacus. 



\ If, leaving California, we cross the Rocky Mountains 



' and 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 



I crayfish, and from Astaats nigrescens, to a much greater 



