THE MOLLUSK FAUNA OF NORTHWEST AMERICA. 247 



more than 24 feet deep. The plateau fauna is remarkable for its fine and singular 

 forms of Chrysodomus, Volutopsius, and other Buccinidse. 



The Arctic or circumpolar mollusk fauna extends to the south as far as the 

 southern limit of floating ice in winter. Many other animals are similarly 

 limited ; the fur seal and the cod keep to the south of this line. 



It is perhaps a little early to do more than roughly indicate the limits bounding 

 the faunal districts of the coast. In the north we have the Aleutian subfauna 

 from the Gulf of Alaska westward ; to the eastward the Oregonian district reaching 

 as far south as Point Concepcion. The Californian, Gulf, and Panamic districts 

 follow, succeeded by the Peruvian and Magellanic. 



On the Asiatic side, the Okhotsk Sea has a special fauna which extends 

 throughout the sea itself and from the southern part of Kamchatka to northern 



Japan, 



On the American coast between the Gulf of Alaska and Dixon Entrance, 



especially in the narrow passages entering the mainland behind the fringing 

 archipelago and fed by many glaciers, are what appear to be more or less isolated 

 colonies of more northern aspect, perhaps remnants of a fauna generally distrib- 

 uted during the colder glacial epoch. 



Coincidently, the warmer waters of the open seas permit a northward extension 

 of southern animals on the outer edge of the archipelago, where a number of 

 species of predominantly more southern habitat have been traced as far north 

 as the entrance to Sitka Sound, though they do not enter the colder waters of 



the Sound itself. 



A striking instance of the importance of sea temperatures in determining 

 the range of marine animals is afforded by the distribution of Turcicula bairdii 

 Dall, a large Trochid which was first dredged by the Albatross in 400 fathoms 

 in the Santa Barbara channel of California, and later found in southern Bering 

 Sea in 25 fathoms; the sea temperature being the same in both cases. 



The Pliocene period here, as on the eastern coast of North America, was 

 characterized by a milder climate than that which immediately succeeded it. 

 Thus the Pliocene beaches of Nome, Alaska, contain a fauna which by the species 

 in it no longer native to that region but now living elsewhere, must have approxi- 

 mated to that at present found in northern Japan and the Aleutian Islands. 

 One might infer that there was also at this time a closer connection with the seas 

 of northeast America than exists at present, since we find fossil in the Pliocene 

 of Sankaty Head, Nantucket, two or three species, which at present live only 

 in Bering Sea. A fine Littorina, of the obtusata type now confined to the Atlantic 

 ocean, is. also found fossil in the Nome Pliocene. The evidence of the beaches 

 indicates a persistent, if not absolutely continuous, moderate elevation of the 

 shore in this vicinity from the Pliocene to the present time. 



If we may take as evidence that a particular area formed a center of distri- 

 bution of certain organic types, the fact that paleontology shows that those 

 types existed in that area during a period geologically long; that at present the 



