i 9 o 5 j ORTMANN— AFFINITIES OF CAMBARUS. 117 



immunis inhabits the (often temporary) shallow, stagnant ponds 

 and roadside ditches of the western prairies, and is a burrower, 

 while C. virilis prefers rocky places in running streams. (See 

 Harris, Americ. Natural., 35, 1901, f. 187 ff., and Kansas Univ. 

 Quart., 9, 1900, pp. 268 and 270). 



Of the other two groups of the third section, that of C. alaba- 

 mensis contains only two species, which are very local, being found 

 only in northern Alabama. Both are rather primitive, and appar- 

 ently are the last remnants in the Tennessee drainage of a once 

 more widely distributed stock. The difficilis -group seems to rep- 

 resent a southern extension of the subgenus Faxonius : the species 

 are found in western Tennessee, Missouri, Arkansas, Indian Ter- 

 ritory, northeastern Texas and Mississippi, all in the drainage of 

 the lower Mississippi (below Cairo), only C. mississippiensis be- 

 longs to the Tombigbee river drainage. 



C. lancifer would agree in its range (Mississippi and Arkansas) 

 with this latter group. 



The species of this subgenus, generally, are river-species, and 

 prefer the large rivers of the great central basin. Some species 

 have become lake-forms ( C. propinquus, for instance), and others 

 ascend the rivers into the smaller streams (chiefly so in the Tennes- 

 see and upper Ohio drainages), but they rarely inhabit true moun- 

 tain streams. 



Further investigation of the distribution of this subgenus should 

 pay particular attention to the ways by which several species have 

 crossed the divides of the Hudson Bay, Great Lakes, and Atlantic 

 coast plain drainage systems. It is very likely that wandering of 

 the divides has played here an important part. 

 Subgenus: Bartonius (Type : C. bartoni). 



This subgenus, which corresponds to the third group of Faxon, 

 is a very natural one, and, in my opinion, contains the most mod- 

 ern and most highly specialized forms in those that have acquired 

 burrowing habits (diogenes-section) . There are, however, other 

 species, which are rather primitive, as indicated by certain char- 

 acters. 



The length of the areola, in this subgenus, is rather variable : in 

 the extraneus-section it is shortest, about half as long as the anterior 

 section of the carapace, and it is even shorter than that in C. acu- 

 minatus. In all other species it is considerably longer. The an- 



