MEXICAN REGION. 109 



muricated (P. magnifica) ; and species of Valvata, Limncea, Physa 

 (15), Planorhis, and Ancylus (5). 



The fresh-water bivalves are also extremely numerous : tlie 

 Unionidce are unequalled for their ponderous solidity, the rich 

 tinting of their interiors, and the variety of their external forms.* 

 Onathodon cuneatus, Cyrena floridana^ 16 species of Cyclas, and 

 Pisidium altik, belong to this region. 



19. Oregon and California. 



The Fauna of the region beyond the Eocky Mountains is 

 believed to be almost entii*ely distinct from that of the United 

 States. Arion (foliolatus) and Limax (Columbianus), genera not 

 indigenous to eastern America, were found near Puget Sound. 

 (Gould). We have no information respecting the land and 

 fresh-water shells of Eussian America, but from analogy we 

 may expect to find a few there identical with those already 

 mentioned as occurring in Siberia, t 



The shells of Oregon and California are principally known by 

 the researches of Nuttall, Couthouy, and Binney. 



Helix 34 Physa 9 Cyrena 2 



Bulimus 10 Ancylus 4 Cyclas I 



Achatina 1 Planorbis 12 Unio 1 



Succinea 4 Melania 2 Alasmodon 1 



Limnaea 12 Potamides 2 Anodon 3 



Limncea fragilis, a Canadian species, is said to range westward 

 to the Pacific ; and L. jugularis to be common to Michigan, the 

 North-west territory, and Oregon. (De Kay.) Limnoea umlrosa, 

 Say ? and PlanorUs corjpulentus, Say, are found in the Columbia 

 Eiver. 



20. Mexican Eegion. 



The lowlands of the northern half of Tropical America con- 

 stitute only one botanical region, extending from the Eio Grande 

 del Norte to the Amazon ; but on zoological grounds it may be 

 divided into two smaller areas. The Mexican province, including 

 Central America, itself comprises three physical regions : the 

 comparatively rainless and treeless districts of the west ; the 

 mountains or high table-lands with their peculiar flora ; and 



* The private cabinet of Mr. Jay contains above 200 species of North American 

 UnixmidcE, and very many varieties. 



t The affinity between the Mammalia of the Old and New Worlds is greatest in 

 eastern Asia and north-west America, and diminishes with distance from thosd 

 regions. ( Waterhouse, in Johnston's Physical Atlas, No. 28.) 



