NOKTHWEST COAST MAKINE SHELLBEARING MOLLUSKS. 5 
The latest previous list for the region, that of Carpenter, enumer- 
ates 468 species. Of the fauna as now enumerated, 136 forms are 
common to the Atlantic waters, nearly all belonging to the Arctic 
seas. There are a few species from the western part of Bering Sea 
which might more properly be assigned to the Okhotsk or Asiatic 
fauna, which is generally quite distinct from that of the American, 
shores, but I have thought it best to include them as they may extend 
to the western Aleutians. The plateau fauna of the shallow part of 
Bering Sea, which is also in the main distinct from that of the sur- 
rounding coasts, has not been segregated from the Aleutian group 
owing to the fact that thorough collections have not been made, and 
in the present state of our knowledge it is impossible to be certain 
how many and which species are exclusively confined to the plateau 
group, which consists largely of Buccinidae. 
The fauna of Kamchatka and the Okhotsk Sea, besides a large 
number of endemic species, contains many analogues of forms 
peculiar to the American coast, but specifically distinct from the 
latter. This is probably due to the chasm of deep water extending 
from the area between the end of the Aleutian chain and the Russian 
possessions. This abyss extends northward east of the Commander 
Islands almost to Bering Strait, heading near the entrance to Plover 
Bay on the Siberian side. The separation of the two areas has ap- 
parently existed long enough for the modification of the American 
and Okhotsk faunas to specific differences in many originally identi- 
cal forms. 
We find also a certain number of species still persisting in the 
region of North Japan, the Kuril Islands, and southern Kamchatka, 
which are also found in the Oregonian and northern California 
faunas, but not in the Aleutian region between their present habitats. 
These are probably relics of the time when warmer seas extended to 
the north and a gradual refrigeration exterminated them in the inter- 
vening area. Such a species is Ilaliotis kamchatkana. 
The Tertiary and Pleistocene fossils of the shores of Bering Sea 
also bear evidence of a communication with Atlantic waters during^ 
the prevalence of more genial conditions. Several species now living 
in Bering Sea are found fossil in the late Pliocene of Sankoty Head, 
Nantucket, and the Pliocene of Iceland, and conversely the common 
periwinkle of New England, Littorina palliata Say, is one of the 
species found in the elevated beaches of the Nome district, Alaska, 
and now extinct on the Pacific side. Evidently the intercommunica- 
tion must have been tolerably free between the two oceans at the 
time, though now there are quite pronounced differences between the 
Greenlandic and the Bering Strait Arctic assemblies of mollusks. 
