75 



LiZAEDS. 



Dipsosaurus. 

 CalUsaurus. 

 Uta. , 

 Phyllodactylus. 



Snakes. 



Trimorphodon. 



Sypsiglena. 



FMmotJiyra. 



Chilomeniscus. 

 These genera coustitute the most characteristic feature of the two 

 faunae, not occurring in any other part of North America. Trimorpho- 

 don, Sypsiglena, and Phyllodactylus are well represented in Mexico. 



Of Batrachians we have, like the Sonoran, Eyla, ScapMopus, and 

 Bufo, but, on the other hand, Plethodon, as in the Pacific and Eastern. 

 Of the fresh- water fish-fauna, nothing is known; the streams are few 

 and small. This region extends northward to the southern boundary 

 of California. 



Among the Invertebrata, the Mollusca present facts of distribution 

 similar in significance to those derived from the study of the Vertebrata. 

 Thus the Eastern, the Middle, and the Pacific districts are plainly 

 marked out in the fresh-water and land Mollusca. To the former are 

 entirely confined the Streptopomatidae and the great majority of the 

 Unionidae, which together constitute more than two-thirds the species 

 of the Nearctic realm. Of land-shells, the great series of toothed snails 

 [Mesodontinae), which embraces many genera and species, is almost con- 

 fined to the Eastern subregion. The same is true of the snails of the 

 group of Gastrodontinae and of the genera Hyalina?iii6. Hygromia. The 

 Central subregion is characterized by its poverty in all that respects 

 Mollusca, while several genera of land-snails are peculiar to the Pacific 

 region, and are largely represented by species there. One hundred of 

 the four hundred land- shells described from the Eegnum Nearcticum be- 

 long to the western coast. Among snails, the genera Aglaja, Arionta, and 

 Polymita are represented by handsome species. Macrocyclis and Bin. 

 neya belong especially to this region. 



As is to be supposed, the Insects indicate a greater number of subdi- 

 visions than the other animals. The fresh-water Crustacea have been but 

 sparingly studied. They seem, however, to have a wide distribution j 

 thus Cojnbarus (craw-fish) and Artemia are found everywhere where 

 physical conditions are suitable. 



