CHARACTERISTIC DEEP-SEA TYPES. — GASTEROPODS. 65 
vegetation, except as a sediment from near the surface, does not 
exist, so that the flesh-eating mollusks of the deep, when within 
reach of pelagie food, or of the carcasses of dead fishes and 
other decaying organie matter, are not obliged to prey upon 
each other to the same extent as do the shallow-water forms. 
The latter take part in a fierce struggle for existence amidst the 
vicissitudes of tidal and storm waves, variation in elevation of 
land, and a vastly denser population of all sorts. Compara- 
tively few of the shells dredged from deep water show the frac- 
tures and injuries so common in shells from littoral dredgings, 
or the drill-holes made by the so-called lingual ribbons, a terri- 
ble boring weapon of enemies of their own kind. Most of the 
enemies of deep-water mollusks are blind, or at any rate can 
have little power of vision for objects not luminous. The ab- 
sence of violent motion in deep water removes any mechanical 
effects of that medium from the category of modifying in- 
fluenees upon the animal. "Thus it is evident that the factors 
affecting the restriction of téndencies to variation in the form, 
color, and seulpture of littoral species are nearly eliminated in 
the abyssal regions; so that we may expect in the deep sea a 
very wide range of variation in form aud sculpture within the 
specific limits of the “flexible " species, and an almost complete 
uniformity over very wide areas of the forms which we may con- 
sider as “inflexible ” species. 
Many of the gasteropods must lead a more or less roving life 
in search of their prey ; others, like Dentalium, live buried in 
ooze. A great number of the mollusks are blind. The lamelli- 
branchs live either buried in the ooze, or on the surface of harder 
bottoms anchored by the byssus. Most of them are stationary, 
though, judging from analogy with some of the shallow-water 
genera, they may be capable of considerable change of locality. 
Those mollusks which subsist upon other animals, with a hard 
covering, so that they have to bore or break their way to their 
food, are much less numerous in the deep sea than those which 
feed upon soft tissues, or kill their living prey by bites with poi- 
sonous fangs. The latter, the Pleurotomidee, outnumber any 
other group of mollusks in the abyssal fauna; they are charac- 
terized by a notch near the junetion of the outer margin of 
