II^TRODUCTION. XXIX 



feature, since the letters inscribed upon them, indicative of 

 a bottom consisting of sand, sandy mud, rock, stones, 

 gravel, muddy gravel, shelly ground, and " coral," i. e., 

 nullipore or vegetable coral, mark as many spaces in which 

 certain forms of Mollusks multiply or prevail in preference 

 to other types. As a general rule univalve testacea and 

 naked Mollusca flourish most upon hard, bivalve testacea 

 upon soft gTound, and this appears to hold true, at least 

 with respect to the species described in this work, through 

 all gradations of depth. 



Besides these several sub-divisions of the floor of the 

 Ocean, there are the high levels of the sea-water itself, 

 inhabited by a small assemblage of Mollusks. The genera 

 lanthina and Spirialis, among our testacea, and our solitary 

 species of Salpa, as well as the curious and anomalous 

 Appendicularia among Tunicata, are inhabitants of this 

 marine atmosphere. All these forms are, however, very 

 local around our coasts. In more southern seas, the waves 

 often swarm with pelagic Mollusca. 



To the two uppermost zones are confined all the repre- 

 sentatives of several genera, whilst the species which live 

 in the lower belts belong to genera which have also mem- 

 bers in the Littoral and Laminarian zones. In the two 

 uppermost zones the great majority of testaceous species 

 is found ; in the second, or Laminarian belt the majority of 

 naked and tunicated Mollusks, In the higher zones many 

 of the species, both of univalve and bivalve testacea, are ex- 

 ceedingly prolific, and their individuals gregarious, so that 

 large numbers of one kind of shell are found assembled to- 

 gether within a limited area, sometimes almost to the exclu- 

 sion of other sorts. The proportion of gregarious species to 

 those of solitary or scattered diftusion is much greater in the 

 littoral than in any of the other belts. All the zones of 



