302 ANNALS NEW YORK ACADEMY OF SCIENCES 



merged land bridges, at a time when the northern crayfish were much 

 more primitive than now, and when, for reasons whicli I do not venture 

 to suggest, the tropics were a more favorable environment than now. 

 The northern crayfish have since evolved into Potamohius and Camharus, 

 the southern specialized into the more divergent Farastacus of South 

 America, Cheraps and Eugceus and Astacopsis of Australia and Tasmania, 

 Paranephrops of New Zealand and ? Fiji and Astacoides of Madagascar. 



Of these southern genera, Astacoides is the nearest to the northern 

 types. This is to be expected, if the southern genera are remnants of a 

 cosmopolitan distribution derived by dispersal from the north ; for the 

 Malagasy genus would be a derivative from Ethiopian crayfish, which 

 would be less remote from the north, and would be correspondingly more 

 advanced than in South America or Australia. As far as the more 

 special distribution of the northern crayfish is concerned, Dr. Ortmann's 

 paper affords data for the following interpretations. 



Two genera are concerned, Camharus of the eastern Sonoran region, 

 and Potamohius (Astacvs of most authors) of the Old World and western 

 Sonoran region. 



In his discussion of the genus Camharus Ortmann states that the 

 more primitive forms of the first, second and fifth groups belong chiefly 

 to the south towards Mexico, and interprets this as meaning that the 

 genus came from Mexico. But, according to the principles here adopted, 

 this should mean that the center of dispersal is to the north and east ; 

 and the discontinuity in range to the south and west is exactly what we 

 should expect. Dr. Ortmann's attempt to find an explanation for it on 

 the opposite theory of migration being curiously complex and unconvinc- 

 ing. The most primitive species occur in such widely divergent points 

 as Mexico and Cuba. 



The more primitive genus Potamohius has a more discontinuous range, 

 in Europe, part of Eastern Asia and Western North America, the Asiatic 

 species being nearest to Camharus (i. e.. highest in development) but 

 parallel, not truly closely related. This, I take it, is correctly interpreted 

 by Ortmann as indicating an Asiatic center of dispersal for this genus. 

 But in place of supposing with Ortmann that Camharus originated from 

 species of Potamohius pushing down southward into Mexico and thence 

 northward again (as Camharus) into the United States, it seems to me 

 that the rational explanation would be to suppose that both genera are 

 the disconnected remnants of a formerly Arctic center of dispersal. This 

 would be first split in two by a progressively unfavorable environment, 

 one division passing down into America east of the Cordilleras, and 

 developing into Camharus, the other part in Asia progressing more 



