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 it 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 

 Pleurotomidse ; 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 Chitonidae 

 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 

 it is only necessary to mention one peculiar group of 

 the family, the genus Cadulus, containing numerous 

 species, all of which are small, polished, pellucid shells. 

 They expand their little tubes to a sort of bulb, more 

 or less prominent, which diminishes before they are 

 completed, so that the calibre of the aperture is smaller Dentalium 

 in the adult than in the young; while in the true periongum. 

 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 Trochidse. These are among the most beautiful 

 of spiral shells, often brilliantly colored, profusely sculptured, 



