PRESENT HISTORY AND DISTRIBUTION 55 
(Ensis) sinking itself well below the surface. Some 
species of Lima and Volsella, on the other hand, 
construct a sort of nest out of all kinds of marine 
refuse, held together by the threads of the byssus. 
The Dentalium, again, buries in the sand, leaving 
only the apex of the shell protruding. 
The quieter waters of the laminarian zones are 
also tenanted by many of the foregoing, but they are 
especially the haunt of the Sea-Slugs (Nudibranchia, 
Plate XII., Figs. 20-28), as well as certain Opistho- 
branchs, like the Sea-Hare (A flysia), and the smaller 
Gastropods, with the Oyster and other Bivalves. 
The yet deeper regions of the coralline and nulli- 
pore zones are the special resort of the large Whelks 
(Buccinum undatum, Neptunea antiqua, etc.), Naticas, 
and other carnivorous Gastropods, of the Scallops 
(Pecten maximus, P. opercularis, etc.), Pinna, many of 
the Venus Shells, and other Bivalves. Here, too, 
the strange Aplacophora frequent the oozy areas. 
Down to a depth of 300 fathoms there is a con- 
siderable number of deep-sea representatives of the 
Mollusca, mostly of the smaller and more delicate 
kinds, while a few abyssal examples are found at 
very great depths. 
So far the extreme depths for each class that have 
been obtained were all recorded on the Challenger 
Expedition. Thus, a Chiton (Leptochiton benthus) 
was dredged at a depth of 2,300 fathoms in the 
Pacific Ocean; of Aplacophora, two immature 
