ASTACIDJE. 15 



the limbs of the claw-legs are in their proportions sometimes very dif- 

 ferent in older individuals, the brachium longer even comparatively, 

 the claw longer and thicker, the fingers more curved and furthermore 

 separated from each other at the bases, while other similar differences are 

 to be seen in the antennas and their appendages, in the shape of the 

 rostrum, and the front margin of the cephalothorax. 



Differences in Sculpture. — The differences in the sculpture and arma- 

 ture are naturally much more considerable. The punctation, the gran- 

 ulation, the flat scales, the thorns, the spines, and the teeth, the margins 

 of the thorax and of the great claws, are little or not at all developed in 

 the younger specimens ; they are a little more visible in the middle- 

 aged animals ; even those that are old and full grown sometimes differ 

 considerably in the degree of development. 



Hairiness. — On the contrary, the hairyness of some parts seems a 

 constant character for species, even in the younger animals. But this 

 character is to be found only in few species, and is not generally 

 important. 



Older /Specimens. — On the whole, it may be said that in the older 

 Astaciclge the form and the sculpture are more strongly exarated ; the 

 claw of the first legs is larger and heavier. But, besides the dimorphism 

 of the males described further on, individuals are to be found in a so- 

 called retrograde or arrested development. 



Retrograde Development. — Older and larger animals are found, which 

 in the degree of sculpture and armature are considerably behind speci- 

 mens of similar or even smaller size, and these have apparently stopped 

 at a lower stage of development. Naturally, these individuals are to be 

 found mostly in the group in which the development of the sculpture 

 and of the great claws is greatest, as in the group of Cambarus aciitus. 



Postabdomcn. — The form of the postabdomen is constant in the differ- 

 ent species ; in the females it is usually broader. The external angles 

 of the postabdominal segments are differently shaped, more or less 

 rounded or acuminated. These differences are specific and constant,, 

 but usually they are not very remarkable, and they are difficult to 

 describe. The "lamina analis," especially the middle lamina, offers 

 specific characters. Its basal part always has two or three teeth on 

 each side of its front margin. But I must remark, that in shape and 

 size, and even in number, the teeth are often variable. Nevertheless, 

 when rich materials are at hand this character is useful. The form and 

 especially the margin of the apical part of the middle lamina are like- 

 wise in some species different, and in some degree constant. I remark, 

 as an example of the difficulty of recognizing occasional accidental 

 variations, that I have seen of Astacus Gambclii but two males, one 

 with the apical margin of the middle lamina exactly rounded, the other 

 exactly notched. I consider the latter as perhaps accidentally altered. 



