1 



HAP. ■ 



376 CHARACTERISTICS OF ABYSSAL MOLLUSC A chap. 



water taken into the gills. The amount of nutriment contained 

 in the bodies of dead Foraminifera is so small that a compara- 

 tively large quantity must be swallowed to keep the vital 

 energies active, and therefore the amount evacuated must be pro- 

 portionately larger also. The abyssal Trochidae, then, and many 

 other genera, sustain themselves by feeding on the ' rain ' of dead 

 animal matter which falls upon the ocean floor, not so much 

 hunting their prey as opening their mouths and eating whatever 

 happens to fall into them. Genera which are normally carni- 

 vorous would appear to do the same. The Pleurotomidae, for 

 instance, are a family markedly characteristic of very deep 

 water. Representatives of the genus which occur in shallower 

 water are known to secure their prey while in the living state. 

 But, according to Dr. Dall, a singularly small proportion of deep- 

 sea MoUusca, as compared with those from the littoral region, 

 show signs of having been drilled or attacked by other Mollusca. 

 This could hardly be the case if the Pleurotomidae retained their 

 predatory habits, since they are more numerous in the great 

 depths than any six other families taken together. It has 

 already been mentioned (p. 186) that a large proportion of 

 deep-sea Mollusca are perfectly blind. 



Amongst other remarkable forms from the great depths may 

 be mentioned Pleurotomaria^ with its singular anal slit (Fig. 269, 

 p. 407) extending in some cases half way round the last whorl. 

 Three or four species of this genus, so characteristic of almost all 

 fossiliferous strata down to the Cambrian, have been obtained in 

 very limited numbers off the West Indies and Japan. Dentaliidae, 

 especially the sub-genus Cadulus^ find a congenial home in the 

 slimy oceanmud. One of the greatest molluscan treasures procured 1 

 by the Challenger was Gruivillea alabastrina Wats., a magnificent i 

 Volute as white as alabaster, 6 J inches long, which was dredged I 

 from 1600 fath. in the South Atlantic, between Marion Island! 

 and the Croze ts. Another very curious form, belonging to the . 

 same family, is Provocator pulcher Wats., a shell about half the f 

 size of Guivillea, of stouter proportions, and with an angulated 

 and patulous mouth. This shell was dredged by the Challenger 

 in comparatively shallow water (105-150 fath.) off Kerguelen 

 Island. Among the Trochidae are the fine new genera Basilissa, 

 Bemhix^ and Gaza. The exploring voyages of the American sur- 

 veying steamer Blake., in the Gulf of Mexico and the Caribbean 



