1902.] AND ANCIENT GEOGRAPHY. 283 



As regards the morphological relations of the five groups, we are 

 to consider first Faxon's view (1885, p. 19), that the species of the 

 first group are morphologically the most primitive ones. He draws 

 this conclusion chiefly from the shape of the male copulatory 

 organs. If we compare, however, certain species of the second 

 group {simulans , mexicanus, cubensis) with those of the first group 

 in this respect, we see that they chiefly differ from the latter only 

 in the smaller number of hooks on the pereiopoda of the male (only 

 on the third pair, not on the third and fourth, as in the first group). 

 On this account I should prefer to regard the species named as the 

 most primitive forms of the genus, although, on the other hand, I 

 agree with Faxon (1885, p. 47) in believing that the other species 

 Of the second group more nearly approach the third group. That 

 the third and fourth groups, compared with the others, are more 

 advanced forms is also my opinion. As the most specialized 

 species I regard those of the third group which have acquired 

 burrowing habits {diogenes, argillicola, dubius). The species of the 

 fifth group differ from ail the rest in the presence of hooks in the 

 second and third pereiopods of the male, and thus I think they 

 represent an early separated side branch. The copulatory organs 

 of the male in this group resemble in certain respects more those of 

 the first and second groups than those of the third and fourth, and 

 the more primitive character of these species is also suggested by 

 the general shape of the body. 



Thus we see that the more primitive forms of the first, second and 

 fifth groups belong chiefly to the South and point distinctly to a 

 connection with Mexico, while among the more advanced and 

 specialized forms of the third and fourth groups this latter connec- 

 tion is hardly expressed or not at all. Their origin and main dis- 

 tribution belong to the more northern parts. 



This points to an origin of the genus in the Southwest, and we 

 believe that the genus came from Mexico and immigrated into the 

 United States in a northeasterly direction. 



A few additional distributional facts tend to support this conclu- 

 sion. It seems that in those groups which possess a large represen- 

 tation in the Southwest the distribution is rather discontinuous. 

 This is most evident with the second group. Now discontinuity in 

 distribution of any animal is very often a sign of the breaking up 

 of a former continuous range by unfavorable physical conditions. 

 In the present case it appears that at a certain time the immigro 



