ASTACID^E. 27 



eminent naturalists these varieties have been taken for nine different 

 species. In the genus Cambarus, the thirty-two known species show 

 comparatively very few varieties. But of three of these, viz. C acutus, 

 C. virilis, and 0. Bartoni, the described varieties differ in a more consid- 

 erable manner ; while perhaps some of them, especially of C Bartoni, 

 will be hereafter recognized as different species. Indeed, the fact is 

 too striking to be overlooked ; here there are few species and many 

 varieties, there many species and few varieties. 



CAMBARUS Erica. 



The question, Is Cambarus a peculiar genus different from Astacus 

 or not, is one of great importance to me as monographer. The his- 

 torical statements already quoted are all that have been made, so far as 

 I know, upon the subject. It would no doubt have been more easy for 

 me to judge of the importance of the generic characters if I had been 

 able to study in the same manner all the species of the old genus Asta- 

 cus. But the materials before me, except for North America, are not 

 sufficient ; some genera are entirely unrepresented, of others only a few 

 specimens are at my disposal. I therefore confess that my judgment 

 upon a division of the old genus Astacus into more genera is not com- 

 pleted ; still, after a rather close examination, I am convinced that 

 Cambarus forms a very good and natural genus, and that, if it be not 

 accepted, a very great part of the actually adopted genera must be 

 equally rejected. 



The differences between Astacus and Cambarus are as follow : — 



1. The general form of Astaci is clumsier, coarser, and more oval. 

 The Cambari are more elongated and more cylindrical. 



2. The absence of the gill on the fifth pair of legs in Cambarus is 

 first quoted by De Haan. Cambarus has seventeen, Astacus eighteen 

 gills. But there is also another difference, not before noticed. In 

 Astacus each pair of gills, except the single one on the fifth set of legs, 

 has a broad, deeply folded membrane, closely fixed behind the most 

 external gill lobe. In Cambarus this membrane is always wanting in the 

 gills on the fourth pair of legs, but exists, as in Astacus, in all the others. 



In the true Astacus all the gills with a folded membrane behind have 

 a basal external bundle of shorter but broader and irregularly placed 

 gill tubes ; these are never to be found in Cambarus. The superior ex- 

 ternal plate of the fifth pair of legs in Cambarus is surrounded by longer 

 featherlike hairs; in Astacus we find but few on the posterior border; 

 C. pettucidus is similarly organized to the true Astacus. 



I may remark that the breadth of the areola or the medial postdor- 

 sal region (Dana) seems not to depend, as it would be easy to suppose, 

 upon the presence or absence of the gills on the fifth pair of legs. We 



