176 BULLETIN OF THE 



interesting species, taken up too late to have figures of them included 

 in the present set of plates, will be illustrated at the completion of the 

 work. 



General Considerations. 



In any account of deep-sea Mollusca it is advisable to premise, first, 

 that our knowledge of them is far from thorough or complete, and con- 

 sequently our conclusions about them must not be considered as final 

 in all cases. Secondly, the conclusions drawn from a study of the Mol- 

 lusca, with their special modes of life and reproduction, are frequently 

 quite different from the results which would follow from a study of 

 other auimals, such as fishes or sea-urchins, whose modes of life and 

 reproduction are widely different from those of moUusks. In short, in 

 drawing general conclusions we cannot include all classes of deep-sea 

 animals as if they formed a homogeneous population. 



There are of course certain features in regard to which general rules 

 apply to all the inhabitants of the deeps, but they are few and liable to 

 modification with greater knowledge. 



In discussing the Blake collections the work done by other expeditions 

 is often important for the proper understanding of the facts developed, 

 and consequently will occasionally be referred to for that purpose. 



The "Challenger" and "Albatross" have both dredged in close prox- 

 imity to some of the Blake stations ; probably the richest haul on the 

 whole Challenger voyage was that obtained near St. Thomas in the 

 West Indies. 



The collection of Mollusca obtained by the parties on the "Blake" 

 was notable in several respects beside those which may be reasonably 

 ascribed to the methods used in collecting. To the latter we may refer 

 the absence or rarity in the collection of very minute forms, which are 

 only accidentally preserved in the contents of a trawl net, even from com- 

 paratively shallow water ; while it is hardly to be expected that, in the 

 long period of washing and straining which the contents of a trawl un- 

 dergo while being hauled in from deep water, anything small enough 

 to go through the meshes of the net should be retained. 



On the other hand, large shells appear to be rare in the great depths, 

 and when found are usually of great fragility ; so that their destruction 

 or serious fracture is almost inevitable. For these or other reasons, 

 deep-sea dredging has afforded few specimens of even moderately large 

 size, judged by the standard of shells living in shallow water or along 

 the shores. Among sheU-les's moUusks several which were of unusual 



