MUSEUM OF COMPARATIVE ZOOLOGY. 185 
glossa, as represented chiefly by the Plewrotomide, outnumber any other 
single group of mollusks in the abyssal fauna. 
.The groups of less specialized character, such as the tooth-shells 
(Dentalium), are rather abundant in species, more so than those of a 
medium character which intervene between them and the highly spe- 
cialized Pleurotomide, but our knowledge of the deep-sea Mollusca is 
yet too imperfect to afford any important generalizations on this score. 
So far as yet determined, the groups systematically lowest in the scale, 
such as the Chitonide, or mail-shells, are rare in deep water, yet the 
representatives of this family found there belong to the more archaic 
sections of their class. » Some very interesting forms of the molluscoid 
Brachiopoda are found in the abyssal region, among them some of the 
largest known species; but as a general rule the number of species is 
small, and bears no comparison to that afforded by the archibenthal 
area. In the early days of deep-sea exploration it was more or less 
confidently anticipated that the deeps would afford specimens of ani- 
mals characteristic of remote geological ages, which might have been 
preserved there, little changed, while their shallow-water relatives 
had perished from the earth. This expectation has been disappointed. 
While there are numerous representatives of forms first made known 
from Tertiary strata and hitherto unknown from shallow water, there 
are not enough of these to characterize the abyssal mollusk fauna as 
archaic in type, — not more, perhaps, than still exist in comparatively 
shallow water ; none so remarkable as the 7’rigonia of austral seas, the 
Pleurotomaria of the Antilles, or the Nautilus of the Spice Islands. 
There is no relation of abyssal species with fossil species of mollusks 
which compares with that between the land and fresh-water faune of 
to-day and those of the Carboniferous and Jurassic strata, whose Unios, 
Physas, and Pupas are hardly more than specifically distinct from still 
existing members of the same genera, I am impelled to insist more 
forcibly on these facts from realizing that, in the reports on the mollusks 
collected by the “ Blake,” as in the lists of those found by the Fish Com- 
mission and by foreign dredging expeditions, many species find a place, 
and attract general attention from intrinsic interest, which are not to 
be counted as true abyssal species. Such are the Plewrotomaria, just 
mentioned, of which two species were found by the “ Blake” in 69-200 
fathoms, and which belong to a group going back almost unchanged to 
the earliest fossiliferous rocks, such as the Cambrian formation. One 
great value of the Blake collection consists in the fact that it contains 
representatives of animals from all depths in the same general area, 
