HARRIS : DIMORPHISM OF CAMBARUS. 51 



time at which exuviation takes place. This point, however, should be 

 determined more definitely. In the spring of 1901, C. immunis ap- 

 peared in the above-mentioned pond early in March. All males col- 

 lected up to April 15 were first-form. On April 20, the pond was 

 again examined and a large number of second-form males, yet soft 

 from exuviation, were found. On April 25, the most of the males 

 taken were second-form, and none of them had yet attained the nor- 

 mal degree of hardness. A large proportion of the first-form males 

 were evidently nearly ready to exuviate. None, however, were ob- 

 served in the act. Upon slipping the old "shell"' from first-form 

 males ready to exuviate, the animals in their new condition were 

 plainly seen to be second-form. Thus, my somewhat more extended 

 observations for C. imnnoiis Hagen confirm Doctor Faxon's impor- 

 tant discovery, that the "first" and "second" forms are simply alter- 

 nating periods in the life of the individual. 



While neither so extended nor careful as for C. imraunis, my ob- 

 servations on the habits of C. virilis led me to believe that the proc- 

 ess is the same in this species. 



Two individuals taken in the above-mentioned pond are deserving 

 of special consideration. The first (No. 34), a specimen of nearly 

 three inches in length, was collected on the morning of April 29, 

 1901, and thrown with many others into a collecting can. In the 

 afternoon, when the material was removed and examined, it was no- 

 ticed that this specimen had died during the process of exuviation. 

 The cast was plainly second-form, as was also the animal in its new 

 condition. The pleopods and the hooks on the ischiopodites were de- 

 cidedly and plainly second-form in both the cast and the exuviated 

 animal. The old chela? resembled those of the second-form, but those 

 of the soft animal were so much distorted that they offered no evi- 

 dence of value. 



The other specimen (No. 35) was collected while very soft from 

 exuviation. It was in every respect a clearly marked first-form male, 

 and, while the cast was not secured, it seems altogether probable that 

 this animal must have passed the winter in the first-form condition. 

 All males taken up to this time in the spring, with the one exception 

 of No. 34, were first-form males, as were all males of this species 

 taken late in the fall. 



C. iminiinis ceases to be constantly second-form long before it at- 

 tains the size of No. 34. No. 35, while a large specimen for C. im- 

 muiiis, was very slightly, if any, larger than other specimens collected 

 in the same place, which changed from first- to .second-form upon ex- 

 uviation. This would seem to indicate that, if there is finally reached 

 a period in the life of the animal in which it is constantly in the first- 



