PALEONTOLOGY: W. H. DALL 
701 
Six hundred miles south of Guadelupe is Socorro Island and its neigh- 
bors, forming the group of the Revillagigedo Islands. It is 330 miles 
west of the Mexican mainland in latitude 18° 50' N., separated by a 
depth of about 1500 fathoms. 
In a general way the sea near the coast of the mainland, west of the 
edge of the continental shelf, is considerably deeper than it is a relatively 
short distance seaward; that is to say a sort of trough in the sea-bottom 
lies near and roughly parallel with the mainshore, for a distance of 
nearly 1000 miles. The soundings are not numerous enough to warrant 
a more definite statement, but are sufficient to indicate the above 
generalization. 
Investigations into the distribution of the Molluscan faunas of the 
Pacific coast of the Americas have been carried on for many years by 
the writer and are gradually reaching a point where positive conclusions 
can be drawn. We find, for instance, the Panamic or tropical fauna 
extending from Point Aguja on the Peruvian coast to Cape St. Lucas 
and the Gulf of CaHfornia, and on the western shore of the peninsula 
of Lower CaHfornia to Cerros Island, and thence (gradually diluted by 
the Califomian fauna) as far as Point Conception, which however is 
reached by extremely few of the really tropical species. 
There is a certain number of littoral mollusks which five only in shal- 
low water near the tidal boundaries and hence are more subject to the 
influences of cHmate than those species which inhabit deeper water. 
Their limits of distribution are strongly marked. 
Now it is a fact that many of these littoral species belonging to the 
Northern or Oregonian fauna actually reach the island of Guadelupe 
though stopping far short of it on the coast of the mainland. 
More surprising still, we find a number of these northern species Hv- 
ing on the island of Socorro, where the normal fauna should be wholly 
tropical: while on Clarion Island in nearly the same latitude and 220 
miles west of Socorro there is Indo-Pacific tropical fauna with conspic- 
uous species like Voluta deshayesii. 
Socorro has, as far as we know, a strictly American tropical fauna 
with little Indo-Pacific adulteration, though it is 110 miles nearer to 
Clarion Island than to the American mainland. The other islands of 
the Revillagigedo group have not been visited by a collector. 
Work has not gone far enough to enable a complete list of the northern 
species now living on Socorro Island, Latitude 18° 20' N., to be made, 
but the following species are included in it. 
Mytilus californianus Conrad. Southernmost reported locality on 
mainland in latitude 24° 40' N., Magdalena Bay, Lower CaHfornia. 
