f 



[From the American Journal of Science, Vol. XXVII, January, 1884.] 



<JM 11 1884/ 



^V 



Art. VIII. — On the so-called Dimorphism in the Genus 

 Gambarus ; by Walter FXtxon. 



The existence of two forms of the adult male in all the 

 species of the-genus Gambarus was discovered by Louis Agassiz 

 and Henry James Clark. The differences between the two 

 forms affect more especially the first pair of abdominal append- 

 ages, organs concerned in the act of coition, but also extend to 

 the general form and sculpture of the body. In one form (un- 

 happily called by Dr. Hagen the "second form "), the first pair 

 of abdominal appendages have a structure nearly like that seen 

 in all young males. The hooks on the third joint of the third 

 (in some species of the third and fourth) pair of legs are small, 

 and in the sculpture of the shell and shape of the claws, this 

 form approaches the female. In the other form (Hagen's "first 

 form "), the articulation near the base of the first pair of abdom- 

 inal appendages is gone and the whole member is much more 

 highly specialized, the terminal hooks being horny, more 

 widely separated and in every way more highly developed ; in 



