78 MAirrAL of tee molltjsca. 



wHIg 34 are peculiar to the Pacific, and 21 to the Atlantic sido 

 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 it is true thai 

 anj' number of living species are common to the Pacific and 

 Atlantic shores, it becomes probable that some portion of thf 

 Isthmus of Darien has been submerged since the Eocene Ter 

 tiary period. Any opening in this barrier would allow the 

 Equatorial current to pass through into the Pacific — there 

 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 Eocky Mountains, were 

 sufiiciently submerged during the Miocene Epoch to allow of 

 the free intermingling of the waters of the Atlantic and Pacific, 

 yet the special temperate moUuscan fauna of E. and W. America 

 are ybtj dissimilar. There are no grounds for believing a single 

 species to be identical. There are, hovrever, 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. Caleforxian Peoyik-ce. 



The shells of Oregon and California have been collected and 

 described by Mr. Hinds, t Mr. Nuttall,J Mr. Couthouy, natu- 

 ralist of the American Exploring Expedition ;§ Mr. Cooper, 

 Dr. Gould, Mr. Binney,|i Dr. Kennerley, Colonel Jewitt, and 

 others.^! 



Bliells common to U. California and SitJm. (Middendorfi''.) 



Littorina moclesta. Troclius atcr. Troclius einyomphalus, 



„ aspera. „ mcBstus. Petricola cjlindiacea. 



Fissurella violacea. „ Fokkesii. Lutraiiu maxima. 



„ aspera . 



* Voyage dans 1" Amerique Mendionole. 1847, t. v. p. v. 



t Voyage of H.M.S. Sulphur ; Zoology by R. B. Hinds, 4to. 1844. 



X Described by T. A. Conrad. Journ. Acad. N. S. Philadelphia, 1834. 



§ Gould in Bost. Nat. Hist. Soc. Proceedings, lS4i5 : and U. S. Exploring Exped- 

 (Commander Wilkes), vol. xii. MoUusca, with Atlas. 4to. Philad. 1852. 



Explorations for a raih'oad route from ihe Jlississippi to tho Pacific Ocean. 1856. 



*[ P. P. Carpenter on Moliasca of West Coast of North America. British Association 

 Eepoi-t for 1863. 



