ASTACID.E. 27 



eminent naturalists these varieties have been taken for nine (lillereut 

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

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

 C. I'ir/'/is, and C. Bnrtoni the described varieties differ in a more consid- 

 cra1)le manner ; while perhaps some of them, especially of G. Bartoiii, 

 Avill be hereafter recognized as diflcrent 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 Erich. 



The question. Is Camharus 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 Asia- 

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

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

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

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

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

 Camharus 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 

 equalh' rejected. 



The differences between Asiacus and Camharus are as follow : — 



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

 The Camhari are more elongated and more cylindrical. 



2. The absence of the gill on the fifth j)air of legs in Camharus is 

 first quoted by De Haan. Camharus has seventeen, Asfacus eighteen 

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

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

 has a bi'oad, deeply folded membrane, closely fixed behind the most 

 external gill lol:)e. In Camharus this membrane is always wanting in the 

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



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

 a basal external ])undle of shorter but broader and irregularly placed 

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

 ternal plate of the fifth pair of legs in Camharus is suri'ounded Ijy longer 

 featherlike hairs ; in Asfacus w^e find but few on the posterior border ; 

 C. pellucidus is similarly organized to the true Asfacus. 



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 



