284 ORTMANN — DISTRIBUTION OF DECAPODS [Aprils, 



tion of Cambarus from Mexico into the United States did not 

 meet with serious obstacles, but that later in the intermediate 

 regions (northern Mexico and Texasj more unfavorable conditions 

 arose which separated the United States more distinctly from 

 Mexico, and this is possibly due to a more decided development of 

 the desert character of these parts. Thus the Mexican representa- 

 tives of the first, second, fourth and fifth groups became more or 

 less separated from those in the United States, the first and fourth 

 groups developed more abundantly in the United States, while the 

 third originated there, possibly out of the second group, which in 

 these parts did not make any marked progress and was suppressed 

 and restricted to a few more or less isolated stations, probably on 

 account of its primitive character. An interesting light is thrown 

 upon this question by the presence of one species of the second 

 group (C cubensis) in Cuba. This species is closely related to C. 

 mexicanus (Pueblo, San Luis Potosi), while it has no closer rela- 

 tions in the United States, and thus its Mexican origin is most dis- 

 tinctly indicated. Therefore we may safely say of the second 

 group that it is a very primitive one and that Mexico, not the 

 United States, is to be taken as its centre of origin. 



The character of discontinuity is more or less noticeable also in 

 the southwestern part of the range of the first, fourth and fifth 

 groups. The first possesses an isolated species {wiegmanni) in 

 Mexico, and the stations of C. blandingi and clarki in Texas are 

 very scattered. In the fourth group we have an isolated species 

 {digueti) in Mexico (Jalisco), while C. immmits, a species found 

 elsewhere in the northern central basin, has been reported from 

 Orizaba, in Mexico.^ The fifth group has two species in Mexico 

 and, widely separated from them, a third near New Orleans. If 

 we compare with this the northern part of the ranges of the first, 

 third and fourth groups we see everywhere perfect continuity. In 

 every direction from the centre, except toward the Southwest, the 

 intensity of distribution decreases gradually. This is especially 

 true for the first group, the centre of which is in the Southern 

 States, in the directions northward along the Atlantic coast and 

 upward in the Mississippi Valley. In the third group, whose centre 

 is in the Allegheny system, there is a regular decrease in intensity 

 in all directions, and in the fourth group a very regular decrease is 



^ We have to accept this record, however, very cautiously. 



