ASTACID^. 89 



ASTACUS. 



Corpore rohisfo ; jicdlhits quiniis bnaichiis ffcrcniibns ; antcnnis infcrnk 

 fdfidh hrci'iori, ina'qudJi ; aurc c<inlc<> post ice apcrto ; pcdihus maris toiiis d 

 (jiKir/is iuermihus ; jjcdidtis ubduiuiiuUibus maris si)iiplicibiis ; fvmimi ai/iiido 

 veufrali solido. 



Having already given the differences of the genera Asfacus and 0am- 

 hariis, I need not here repeat them. 



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

 fifth pair of leg.s has a gill, but without the broad, dee2:»ly 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 bases thick, the joints more spherical and cal- 

 careous. The exterior antenna? are shorter than the bodj-; 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 in the nrale form a corneous, not articulated limlj, 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 leg.s, 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 l^ehind 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 Camiarns, but 

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

 is known, and nothing of jjurrowing hal)its in the species. It seems 

 striking, as already mentioned, that the species of Asiaciis, esjiecially 

 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 I expressly remark, that none of the characters set forth as 

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

 separate the American species. 



12 



