MUSEUM OF COMPARATIVE ZOOLOGY. 185 



glossa, as represented chiefly by the PleurotomidcB, outnumber any other 

 single group of raoUusks 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 Pleurotomidce, but oiir knowledge of the deep-sea MoUusca 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 Ckitonidce, 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 Trigonia of austral seas, the 

 Pleurotomaria of the Antilles, or the Navtilus of the Spice Islands. 

 There is no relation of abyssal species with fossil species of moUusks 

 which compares with th"at between the land and fresh-water faunae 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 raollusks 

 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 Pleurotomaria, 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 

 gi'eat value of the Blake collection consists in the fact that it contains 

 representatives of animals from aU depths ;n the same general area, 



