270 J. A. G. REHN 



adjacent Canada, and some have developed well-marked subsidiary 

 evolutionary centers there. Trimerotropis, an entity with a consider- 

 able number of North American species, is probably a relatively old 

 type, which has extended locally into boreal conditions, has formed 

 certain localized species in the eastern United States, and has further 

 intruded itself southward, so that today the genus is also present in 

 semi-arid and Andean areas of western South America. Its range 

 there is now cut off from the southern limits of its mass distribution 

 at the southern edge of the Mexican tableland. This discontinuous 

 distribution probably exemplifies a far broader and drier Pliocene 

 grassland distribution, and reflects the increased Pleistocene develop- 

 ment of forest areas in the intervening territory, which doubtless 

 eliminated Trimerotropis from Central America and parts of north- 

 western South America. A similar postulate would explain the 

 present discontinuous distribution of the grassland mantid Brunneria 

 and the acridine grasshopper genus Dichromorpha. 



The genus Chortophaga is probably of southeastern origin, there 

 showing two types of the genus, one species of which, present over 

 much of the eastern and central United States, is also found in the 

 more temperate parts of Mexico and Central America as far south 

 as Costa Rica. The endemic Californian genus Chimarocephala is 

 rather an anomaly, although its ancestral stock may have had a 

 common origin with Chortophaga. The genus Cammda probably 

 developed from the same stock as Hippisciis or Encoptolophus, 

 possibly in the Cordilleran region, but it now has an unusual type of 

 distribution; it is essentially Boreal in the eastern and central 

 United States and Canada, broadly present in the Cordilleran 

 region, and much more localized westward (in southern California 

 it even descends to virtual sea level in coastal Lower Sonoran 

 conditions). The broadly spread but often localized genus Xaiithip- 

 piis, a close relative of Hippiscus, apparently developed in the 

 Sonoran region of our Great Plains, the southwestern United States, 

 and northern Mexico, with numerous localized and seasonally limited 

 montane forms, but it has not spread eastward, while the related 

 Cratypedes is much more definitely an inhabitant of the Cordilleran 

 and Great Basin areas. The genera Sticthippus and Agymnastus are 

 Californian endemics, clearly derived originally from the same basic 

 stock as Hippiscus, Pardalophora, and Xanthippus. The genera 

 Leprus and Derotmema are certainly of Sonoran origin, and both 



