248 BULLETIN 15 4, UNITED STATES NATIONAL MUSEUM 



The three forms of the group which occur on islands in the Gulf 

 of California {caeinileu.^, pictus, and danhebtiae) are clearly direct 

 derivatives from a connnon ancestral stock which was much like the 

 modern hypenjthrus. Very shallow soundings (usually much less 

 than 60 fatlioms or 8G0 feet) show that the close lying islands (Car- 

 men, Monserrate, and San Jose) upon which those variants are found 

 are a part of the Lower Californian land mass, so there can be no 

 doubt of the comparatively recent isolation of these small geo- 

 graphical areas. Both Nelson (1922) and Schmidt (1922) have 

 given comprehensive reviews of the available geological data for 



Figure 34. — Map showing distribctiox of the forms of the HrrERTTHRUs aRoup 



Lower California. The Miocene age was presumably one of great 

 igneous activity and following this period the general land surface 

 of much of Lower California has been very irregular. The avail- 

 able evidence indicates an extensive submergence of this area during 

 the post-Pliocene period. This is held to have isolated the Cape 

 ]!)istrict from the northern section of Lower California. If the 

 hyperythms stock had populated the peninsula previous to this sub- 

 mergence, it might be expected that a Cape race would have been 

 differentiated during this period of isolation. (See remarks con- 

 cerning the origin of maxhnvs, p. 221.) But, since hyjieryfhms 



