THE STONE CRAYFISH AND THE NOBLE CRAYFISH. 295 



the post-orbital ridge, and the absence of spines on its 

 surface. 



4. The large relative size of the posterior division of 

 the telson (G). 



On the contrary, in the noble crajfish : — 



1 . The sides of the posterior two - thirds of the 

 rostrum are nearly parallel, and the lateral spines are 

 fully a third of the length of the rostrum from its point ; 

 the distance between them being much less than their 

 distance from the apex of the rostrum (B). 



2. No spine is developed from the ventral margin of 

 the rostrum. 



3. The posterior part of the post-orbital ridge is a 

 more or less distinct, sometimes spinous elevation. 



4. The posterior division of the telson is smaller 

 relatively to the anterior division (H). 



I may add that I have found three rudimentary pleuro- 

 branchise in the noble crayfish, and never more than two 

 in the stone crayfish. 



In order to ascertain whether no crayfish exist in 

 which the characters of the parts here referred to are 

 intermediate between those defined, it would be neces- 

 sary to examine numerous examples of each kind of cray- 

 fish from all parts of the areas which they respectively 

 inhabit. This has been done to some extent, but by no 

 means thoroughly ; and I think that all that can be safely 

 said, at present, is that the existence of intermediate 

 forms is not proven. But, whatever the constancy of the 



