ASTACID.E. 21 



the sexual aperture is visible at the base of the third set of legs. The 

 first abdominal segment is without any appearance of abdominal legs : 

 in all the other segments the abdominal legs are well developed, their 

 length being nearly two thirds of the breadth of the postabdomen, the 

 basal article being oblong, while the length of the doubled fiagellum is 

 a little greater. 



In the males the first segment has on each side a little knob, some- 

 what longer than broad, turning inward. In the interior the developing 

 leg is visible, and its articulation seems marked. This oval knob, with 

 rounded tip, is the beginning of the first pair of abdominal legs. 1 have 

 seen the same form of the first abdominal legs in the young of C. Bar- 

 tonii, even 0.55 inches long. Tab. II. Figs. 133, 134. 



Second Form of the Males. — I have examined the further development 

 in Cambarus acutus. In the younger specimens, 1.7 inch long, the legs 

 are more developed, the basal third articulated. The shape of the legs 

 is nearly the same as in the full-grown animal, but narrower, more 

 curved, the tip a little broader, the teeth more obtuse. In the older 

 specimens. 2.7 inches long, the legs are the same as in the full-grown 

 animal, which is over four inches long. Occasionally, both in younger 

 and in full-grown specimens, the articulation is partly gone, but its 

 remains are still visible on the upper margin ; in some cases the 

 articulation has entirely disappeared. 



This form, which is always visible in the very voting and in middle- 

 aged specimens, ordinarily with an articulation, 1 have described as the 

 second form of the male. 



First Form of the Mates. — In all species seen and examined by me, 

 many old. full-grown males have the first pair of legs of a particular 

 shape, analogous in a certain view to the form before described, and 

 always found in the young animals, but differing in the following 

 particulars. 



The articulation is entirely gone ; the tip of the leg is more dis- 

 tinctly finished and not so membraneous ; the hooks are horny ; the 

 teeth or bifid ends longer and more separated ; the hairvness. if any 

 exists, more profuse. I have figured these parts in nearly all the 

 species, and described them as the first form of the male. I think it is 

 well to observe that, in the second form of the males, they are always 

 developed in a manner visibly less complete than in the first form. 



The males of the second form differ also in another way from those 

 of the first form. The hooks on the third article of the third, or in some 

 groups of the third and of the fourth, pair of legs are smaller and less 

 developed. The whole body has less size and width, the sculpture is 

 not so well finished, while the claws are shorter, narrower, and more 

 like those of the female. 



A closer examination of the rich materials at the Museum shows that 



