VERMES AND MOLLUSC A OF THE DEEP SEA 119 



The Lamellibrancliiata, or bivalves, occur in al- 

 most all depths of the ocean, Callocardia pacifica and 

 Callocardta atlantica having been found at the enor- 

 mous depth of 2,900 fathoms. Some species, such as 

 Venus mesodesma, have a very wide bathymetrical 

 distribution, but others are only known to occur in 

 deep water. 



Concerning the characters of the deep-sea 

 Lamellibranchiates, Mr. Smith, in his report on the 

 Lamellibranchia of the 'Challenger' expedition, says 

 ' very deep-water benthal species certainly have 

 a tendency to be without colour, and of thin struc- 

 ture, no doubt resulting from the absence of light 

 the difficulty of secreting lime, the scarcity of food 

 and other unfavourable conditions of existence.' But 

 notwithstanding this, the same author continues : ' The 

 species are apparently few in number in comparison 

 with those of shallow water ; and new and peculiar 

 generic forms which we naturally expected would 

 have been discovered are of even still rarer occur- 

 rence.' 



As in the group of the Lamellibranchia, so in 

 the Gasteropoda, no very remarkable new genera or 

 species have been found in deep sea. Some shallow- 

 water genera, such as Fiisus for example, have repre- 



