astacidyE. 89 



ASTACUS. 



Corpore robusto ; pedibus quint!* branehus gerentibus ; antennis interrds 

 flagelh breviori, incequdli; aure conico postice aperio ; pedibus maris tertiis et 

 quariis inermibus ; pedibus abdominalibus maris simplieibus ; feinma mi, nth 

 ventraU soKdo. 



Having already given the differences of the genera Astacus and Cam- 

 barus, I need not here repeat them. 



In its general form the species of Astacus are clumsy and oval. The 

 fifth pair of legs has a gill, but without the broad, deeply folded mem- 

 brane peculiar to the gills of all the other legs, •which possess also a basal 

 external bundle of shorter and irregularly placed gill-tubes. The inner 

 antennae are short, their liases thick, the joints more spherical and cal- 

 careous. The exterior antennae are shorter than the body; their lamina 

 is prismatic, being more thickened on the external border. The epistoma 

 is solid, conical, a little contracted in front of the tip. The ear forms an 

 elevated cone, rounded at the top, with a narrower circular tympanum 

 behind. The areola is broad and slightly marked. The postabdomen 

 is always broad, the exterior angles of the segments are often elongated 

 and acuminated. The third and the fourth pair of legs in the males 

 never differ from the other legs, and are never hooked. The first ab- 

 dominal legs hi the male form a corneous, not articulated limb, with the 

 apical half dilated and rolled from the outside inward, forming also a 

 channel. The shape of these legs seems not to vary in the different 

 species, at least no difference is as yet known. In the second pair of 

 abdominal legs the inner flagellum with the dilated basal half is rolled 

 from the inside outward, or it has exactly the form of that of the first 

 abdominal legs, as in the European species, or it is of a more triangu- 

 lar shape, similar to the Cambarus, as in the American species. The 

 separated and perforated annulus ventralis behind the fourth pair 

 of legs of the females, described in Cambarus, is not to be found in 

 Astacus. In fact, the same part exists here, though in the European 

 species it is never separated, but forms only a slender transverse ridge, 

 which in the American species is curved behind like a horseshoe. In 

 the American species it is far more dilated behind in a triangular man- 

 ner, excavated beneath, and apparently more similar to Cambarus, but 

 neither separated nor perforated. As yet no dimorphism of the males 

 is known, and nothing of burrowing habits in the species. It seems 

 striking, as already mentioned, that the species of Astacus, especially 

 those from Europe, offer so many varieties, which are rarely found, con- 

 sidering the great number of species in the American Cambarus. At the 

 same time 1 expressly remark, that none of the characters set forth as 

 variable in the European sjiecies is used by me to characterize and to 

 separate the American species. 



