CHARACTERISTIC DEEP-SEA TYPES. — GASTEROPODS. 67 
extremely thin, with peculiar flexuous growth lines and no oper- 
culum. The variety in this group seems endless, and in num- 
ber of species 1t is likely to rival even some of the great groups 
of land shells. 
The groups of less specialized character, such as the tusk- 
shells (Dentalium), are rather abundant in species, more so than 
those which intervene between them and the highly specialized 
Pleurotomide ; but our knowledge of the deep-sea mollusks is 
yet too imperfect to afford any important generaliza- 
tions on this score. So far as determined, the groups 
systematically lowest in the scale, like the Chitonidee 
or mail-shells, are rare in deep water, yet the deep- 
sea representatives of this family belong to the more 
archaic sections of their class. The tusk-shells are 
curved tubes, almost all white or delicately tinted, 
and varying chiefly in curvature, calibre, and super- 
ficial sculpture or color. The most remarkable of 
these, among the slender species, is Dentalium per- 
longum (Fig. 284), polished, white, nearly smooth, 
and attaining a greater relative length than any other 
species, over four inches, with a diameter of an eighth 
of an inch at one end, and half as much or less at the 
other. It reaches the greatest depths dredged by the 
* Blake" (over 2,000 fathoms), and has not appeared 
in shallow water. There are many other species, but 
itis only necessary to mention one peeuliar group of 
the family, the genus Cadulus, containing numerous 
species, all of which are small, polished, pellueid shells. 
They expand their little tubes to a sort of bulb, more 
or less prominent, which diminishes before they are E 
completed, so that the calibre of the aperture is smaller dei 
in the adult than in the young; while in the true Perlongum. 
5) 
Dentalium the diameter gradually increases with age. 
The Caduli are quite characteristic of the deeper waters of 
the sea. 
Another group also largely represented in the abyssal region 
is that of the Trochide. These are among the most beautiful 
of spiral shells, often brilliantly colored, profusely sculptured, 
