BATHYMETRICAL DISTRIBUTION OF MARINE MOLLUSKS. Ill 



tions. This zone is inhabited b}'- many of the predacious genera 

 — Buccinum, Fusus, Pleurotoma, ISJ^atica, Aporrhais, Philine, 

 Yelutina ; and by vegetable feeders, such as Fissurella, Emarg- 

 inula, Pileopsis, Eulima, and Cheuinitzia. The great banks of 

 scallops belong to the shallower part of this region, and many 

 bivalves of the genera Lima, Area, Nucula, Astarte, Yenus, 

 Artemis, and Corbula. 



4. Deep-sea Zone. From 50 to 100 fathoms the Nullipore still 

 abounds, and small branching corals to which the Terebratulte 

 adhere. In northern seas the largest corals (Oculina and 

 Primnoa) are found in this zone, and shells are relatively' more 

 abundant, owing to the uniformity of temperature at these 

 depths. These deep-water shells are most\y small and destitute 

 of bright colors ; but interesting from the circumstances under 

 which they are found, their wide range, and high antiquity. 

 Amongst the characteristic genera are Crania, Thetis, Nesera, 

 Cryptodon, Yoldia, Dentalium, and Scissurella. In the mud 

 brought up from deep water may be often found the shells of 

 Pteropoda, and other mollusca which live at the surface of the 

 sea. 



5. Abyssal Zone. The fauna of this zone and its distribution 

 are as yet hardly known. MoUusks continue abundant at from 

 800 to 1200 fathoms, but the number of species appears to be 

 small. At the depth of 2435 fathoms, the Porcupine Expedition 

 obtained five species, and at 2*740 fathoms the Challenger Expedi- 

 tion dredged three pelecypods, Area, Limopsis, and Leda. 



Usually the mollusks of this zone are of small size, not highl}^ 

 colored,' with a white, thin and translucent shell. The visual 

 organs are rarely deprived of pigment ; Eulima stenostoma, 

 Pleurotoma nivalis, and Pecten fragilis are stated to be blind. 



The dominant forms of great depths are the Scaphopoda (Denta- 

 lium, Siphodentalium, Cadulus),the tetrabranchiate Gastropoda 

 (Philine, Cylichua, TJtriculus, Scaphander, ActtBon,some species 

 of Pleurotoma and Fusus), a few of the rhipidoglossata 

 (Scissurella, Puiicturella, Cyclosti-ema, Seguenzia', and some 

 lamellibranchs (Area, Limopsis, Nucula, Leda, Malletia, Lima, 

 Pecten, Amussium, Dacrydium, Axinus, Pecchiolia, Syndosmya). 

 Brachiopods are also found ; one species, Terehi^atula Wyvillei, 

 at a depth of 3000 fathoms. 



Wyville Thomson has compared these abyssal shells with the 

 cretaceous fauna, but they are not at all related, resembling 

 rather the pliocene fauna of Italy, and the recent fauna of less 

 depth in the Arctic and Antarctic seas — where analogous 

 temperature prevails, and whence the species may have immi- 

 grated. 



In 1863, S. Loven emitted the opinion that a fauna of identical 

 character extends from pole to pole, traversing all the degrees 



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