78 MANUAL OF THE MOLLUSCA. 
while 34 are peculiar to the Pacific, and 21 to the Atlantic side, 
of S. America; an extraordinary amount of diversity, attribut- 
able partly to the different character of the two coasts—the 
eastern low, sandy or muddy; the western rocky, with deep 
water near the shore.* 
The comparison of the shells of Eastern and Western America 
is of considerable interest to geologists; for if 1t is true that 
any number of living species are common to the Pacific and 
Atlantic shores, it becomes probable that some portion of thr 
Isthmus of Darien has been submerged since the Eocene Tez 
tiary period. Any opening in this barrier would allow the 
Equatorial current to pass through into the Pacific—thero 
would be no more Gulf stream—and the climate of Britain 
might, from this cause alone, become like that of Newfoundland 
at the present day. 
Although geological researches seem to show that not only 
the Isthmus of Darien, but even the Rocky Mountains, were 
sufficiently submerged during the Miocene Epoch to allow of 
the free intermingling of the waters of the Atlantic and Pacific, 
yet the special temperate molluscan fauna of E. and W. America 
are very dissimilar. There are no grounds for believing a single 
species to be identical. There are, however, a large number of 
species (upwards of 50) living on both sides of the northern por- 
tion of the continent, and the majority of these exist in the 
British seas. 
XII. CALIFORNIAN PROVINCE. 
The shells of Oregon and California have been collected and 
described by Mr. Hinds,+ Mr. Nuttall,t Mr. Couthouy, natu- 
ralist of the American Exploring Expedition ;§ Mr. Cooper, 
Dr. Gould, Mr. Binney,|} Dr. Kennerley, Colonel Jewitt, and 
others.&) : 
Shells common to U. California and Sitka. (Middendorfi.) 
Littorina modesta. Trochus ater. Trochus euryomphalus, 
ss aspera. >>  meoestus. Petricola cylindracea. 
Fissurella violacea. »  Fokkesii. Lutraria maxima. 
sp aspera. 
* Voyage dans l’ Amérique Méridionale. 1847, t. v. p. v. 
+ Voyage of H.M.S. Sulphur ; Zoology by R. B. Hinds, 4to. 1844. 
¢ Described by T. A. Conrad, Journ. Acad. N. S. Philadelphia, 1834. 
§ Gould in Bost. Nat. Hist. Soc. Proceedings, 1846; and U.S. Exploring Exped. 
(Commander Wilkes), vol. xii. Mollusca, with Atlas. 4to. Philad. 1852, 
| Explorations for a railroad route from the Mississippi to the Pacific Ovean. 1856. 
| P. P. Carpenter on Mollusca of West Coast of North America. British Association 
Report for 1863, 
