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 crayfish :— 
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- 
branchie in the noble crayfish, and never more eee 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 
