88 BULLETIN OF THE 



tendency to variation is less dependent upon changes in the existing 

 environment than has generally been assumed. 



The total number of literal species recorded by Adams and D'Orbigny, 

 throwing out those groups, like the Pteropods, not germane to the 

 inquiry, is 580, as compared with 461 collected by the Blake. The 

 number of genera represented by the former is about 110, while some 

 98 genera are found in the Blake collection. The 461 species included 

 in the last-mentioned collection comprise 210 which are litoral or do not 

 reach great depths, while 251 are abyssal or ubiquitous. These numbers 

 are of course approximate, and subject to correction, but probably not 

 seriously in error. 



Out of 48 species, of 44 genera, having great vertical range, 24 have a 

 range of 500 to 750 fathoms; 17 have a range of from 750 to 1,000 

 fathoms; and 7 have a range of from 1,000 to 1,555 fathoms. Bearing 

 in mind that the absolute depth of the extreme range may be much 

 greater than this, the astonishing fact is evident that the same species 

 may experience a difference, between two of its stations, of the weight of 

 nearly two miles of sea-water. The possibility of this of course lies in 

 the permeation of the soft parts by the sea-water, thus equalizing the 

 pressure. It is almost certain, however, that individuals from the great 

 depths would die if removed to shoaler water, unless by extremely slow 

 degrees. 



It is noticeable among the deep-sea forms that the sculpture tends to 

 be slight, the shell thin, pale or colorless, and in the spiral shells there 

 is a tendency to a knobbing or denticulation of the posterior edge of the 

 whorls at the suture. To each of these peculiarities there are, however, 

 conspicuous exceptions. 



The following tables exhibit in detail the statistics from which the 

 foregoing conclusions have been drawn. 



