28 ASTACIDiE. 



find in some Cambari (C. spiculifer, versutus) this areola even as broad as 

 in many true Astaci with gills [A. Klamathensis, Gambeli), but in general 

 the areola is never so well marked in Astacus as in Cambarus. 



3. The inner antennae in Astacus have a peculiar structure and shape. 

 They are always very short and more conical (the basis thicker) ; the 

 inner flagellum is considerably more slender and shorter, while the 

 joints of the flagellum are more spherical, calcareous, and more fragile. 

 Of course the inner antennae are easily broken in the preserved speci- 

 mens. In Cambarus the inner antennae are visibly longer, the flagellum 

 is equally long, and of the same structure as the outer antennae. 



The lamina of the outer antennae has a prismatic shape in Astacus, the 

 external border is much thickened. In Cambarus the lamina is visibly 

 more membranous. 



The basal article of the inner antennae has an anteapical sj)ine be- 

 neath in Astacus ; in Cambarus this spine is always situated in the middle 

 of the article, or more basally. 



4. The epistoma in the true Astacus is more solid, conical, a little con- 

 tracted before the tip. It is in Cambarus more flattened, often exca- 

 vated beneath, always larger, and never contracted before the tip. 



5. The ear, or what is considered the auditory organ, forms in Cam- 

 barus (as in Astacoicles, Homarus, and perhaps in Clieraps) a very short cone 

 or a slightly elevated ring, closed above by a membrane, considered as 

 the tympanum. The true Astaci are an exception, and have this organ 

 differently shaped, with a more elevated cone, rounded on the top, and a 

 narrower tympanum behind. 



6. The parts which serve for sexual purposes in Cambarus differ 

 essentially from those in Astacus. In Cambarus these parts are organ- 

 ized in a particular manner, and differently in every species. This is 

 very important, as it is well known that in the Articulata very nearly 

 allied genera often differ constantly in such a manner. This difference 

 seems a criterion for separating two genera in forms otherwise nearly 

 related. 



These differences are as follow : Dimorphism is to be found in the 

 males. The first pair of abdominal legs is differently formed ; the api- 

 cal half is not simply rolled as in Astacus, but transformed into two solid 

 approximated parts, with the tips more or less protracted and differ- 

 ently finished. 



In the females is to be found behind the sternum, between the fourth 

 (and fifth) pair of legs, a particular separated part, — annulus, — differ- 

 ently shaped in the different species. In Astacus this part exists in- 

 deed, but it is never separated from the sternum, and is represented by 

 a ridge, either straight and transverse or curved and broken behind. 

 The particular shape of the annulus in Cambarus, with its denticulated 

 median suture and its transverse hollow impression, is evidently anal- 



