ASTACID^E. 109 



second, doubtless comprise several subordinate faunal districts. The 

 region watered by the Mississippi apparently divides itself into two 

 parts, a southern and a northern, the latter beginning near the mouth 

 of the Ohio River. The eastern region has a decided and peculiar 

 southern fauna, which is perhaps to be united with that of Cuba, and a 

 northern fauna beginning in Pennsylvania. In the northern region 

 there seems hardly any difference between the eastern and western 

 parts. Nevertheless, it is certain that some species belong to more 

 than one region, and that these regions do not coincide with the three 

 principal groups of the genus Cambams. The first group, as has been 

 already stated, prevails in the southern parts of the western and eastern 

 faunal region ; the second group prevails partly in the northern part of 

 the eastern faunal region. 



The examination of the distribution of single species, or rather the 

 exact determination of the circle in which every species lives and the 

 central point which is to be taken as its most proper habitat, would 

 be very interesting, if the materials at command were sufficiently abun- 

 dant. The greatest impediment to such an examination is the scanti- 

 ness of our knowledge, I may say our almost entire ignorance, of the 

 great country comprising the Alleghany Mountains and circumjacent 

 regions. 



A detailed examination of the questions suggested being as yet im- 

 possible, I may be permitted to offer a few words upon some of the most 

 widely spread species. 



The central point of C. acutus seems to be the southern shore around 

 the mouth of the Mississippi ; of C. affinis, the Lower Potomac ; of C. 

 virilis, Lake Superior ; of C. propinquus, Lake Ontario ; of C. obesits, the 

 middle part of the Mississippi ; of C. Bartonii, perhaps the middle of the 

 eastern part of the United States. Some of these so-called central 

 points are in fact not at all central, they being either near the shore of 

 the sea or not far from the limits, so far as we yet know, of the distri- 

 bution of the species. I accordingly attach no great value to them, as 

 they are liable very soon to be greatly modified by new explorations. 



The number of species now known to belong to the old genus Astaais 

 is 56. Of these there are in America, 40 (two in South America) ; New 

 Holland, 11 ; Asia, 2 ; Europe, 2 ; Africa, 1. 



As now distributed, there belong to the genus Cambams, 32 ; Astacus, 

 13; (Astacoides) Astacoides, 4; Cheraj)s,l; Engams,2. The systematic 

 position of the other species is not yet ascertained. 



