MEXICAN REGION. 109 
muricated (P. magnifica); and species of Valvata, Limncea, Physa 
(15), Planorbis, and Ancylus (5). 
The fresh-water bivalves are also extremely numerous: the 
Unionide are unequalled for their ponderous solidity, the rich 
tinting of their interiors, and the variety of their external forms.* 
Gnathodon cuneatus, Cyrena floridana, 16 species of Cyclas, and 
Pisidium altile, belong to this region. 
19. OREGON AND CALIFORNIA. 
The Fauna of the region beyond the Rocky Mountains is 
believed to be almost entirely 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 haye no information respecting the land and 
fresh-water shells of Russian America, but from analogy we 
may expect to find a few there identical with those already 
mentioned as occurring in Siberia.+ 
The shells of Oregon and California are principally known by 
the researches of Nuttall, Couthouy, and Binney. 
PENCE Keaeccerscecesvervecess SHA VPHYSaiaccacsseeccnsatncedss 9 (OAyaHEIME), Gooncocdndringonate 2 
HS TUITIUSS ss. <aecoesoressos 10 ATIC VIUS) ces sccsseeseccnss 4 CY. ClAaS i acenen coeoneerace 1 
IAGHALING)” setsecccordeese 1 PIANOLDISi wecscsscusesere 1 ieee Of obo iemnaneeorcriopccpce atic 1 
HECINGAL st ssccswcrteues 4 IVICTAMI A erneccssacescee.’ ee Alasmodon .........06. 1 
MANA es sas ceeest ence 12 Potamides....... eoaeaane 2 Anodoneeswedehecnesee 3 
Limnea fragilis, a Canadian species, is said to range westward 
to the Pacific; and Z. jugularis to be common to Michigan, the 
North-west territory, and Oregon. (De Kay.) Limncea wmbrosa, 
Say ? and Planorbis corpulentus, Say, are found in the Columbia 
River. 
20. Mexican REGION. 
The lowlands of the northern half of Tropical America con- 
stitute only one botanical region, extending from the Rio 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 
Unionide, and very many varieties. 
} 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 those 
regions. ( Waterhouse, in Johnston’s Physical Atlas, No. 28.) 
