io6o PRINCIPLES OF STRATIGRAPHY 



sidered. The first of these genera includes 67 species, and is 

 found only in the eastern parts of North America, Mexico, and 

 Cuba. The genus is divisible into five groups, the smallest of 

 which contains three species and the largest twenty-six. The first • 

 group of sixteen species is restricted chiefly to the southern parts 

 of the United States and Mexico. The most primitive species belong 

 chiefly to the southwest ; and in fact this is also true of the primitive 

 species of the second and fifth groups. This fact points to an origin 

 of the genus in the southwest, its starting point being apparently in 

 Mexico. The second genetic group, with eiglit species, shows a 

 striking discontinuity of distribution, isolated representatives being 

 found in Mexico, the Gulf States, the southern Atlantic States, 

 and Cuba ; all of them separated from the members of the group 

 found in the southwestern and central States. This indicates that, 

 after the migration of the species of this group from Mexico into 

 the United States, unfavorable physical conditions at intermediate 

 points broke up the former continuous range. The third group, 

 with thirteen species, has a continuous distribution in the Alle- 

 ghany mountains and in the east ; a single species reaching north to 

 the Gulf of St. Lawrence region. It also extends northward in 

 the Mississippi region, but is less prominent, and is represented at 

 only one station in the southwest, and there by a single species. 

 The fourth group, with twenty-six species, is also scarce in the 

 south and southwest, but is abundant in the Upper Mississippi- 

 Missouri-Ohio basin. Eastward it extends up the Ohio into Penn- 

 sylvania, Virginia, Maryland, New Jersey, and New York, its 

 northeastern limit being near Montreal. In the central region it 

 extends northward to Lake Winnipeg and the Saskatchewan River, 

 the most northerly locality known for this genus. Westward it 

 extends to Wyoming. The fifth group consists of three species, one 

 occurring near New Orleans, the other two in Mexico. On the 

 whole, the most primitive species are in the Mexican region, the 

 probable birthplace of the genus, as has already been suggested. 

 Here also has remained a rather primitive side branch, group 5. 

 The other groups advanced northward and northeastward, the 

 most specialized becoming discontinuous on account of adaptation 

 to a changed environment. 



Cambarus appears to be derived directly from a less specialized 

 fresh water crayfish, Potamobius, which has a strikingly discon- 

 tinuous distribution, one group of seven species occupying a con- 

 tinuous area in Europe and western Asia, and another in western 

 North America. A third group, separated as the subgenus^ Cam- 

 baroides, occurs in northeastern Asia: this group, according to 



