380 THE MEDALS OF CREATION. Cuap. XI. 
Some tribes of testaceous mollusca are exclusively marine ; 
many are restricted to the brackish water of estuaries ; others 
live only in fresh-water; and some on the land. Their 
geographical distribution is alike various: certain groups 
inhabit deep water only, and are provided with means by 
which they can maintain themselves near the surface of the 
ocean, far away from any shore ; while others are littoral, 
that is, live in the shallows along the sea-shores. Many 
exist in quiet, others in turbulent waters; some are grega- 
rious, like the oyster ; while others occur singly, or in groups. 
The vertical range, that is, the relative depths in which the 
mollusca live in the sea, is also strictly defined ; certain» 
genera being, in a great measure, restricted to moderate 
depths, others to a few fathoms, and many to the profound 
abysses of the ocean, which neither the dredge nor the 
plummet can reach. All these varieties of condition are 
more or less strongly impressed on the shells, which may be 
considered as external skeletons ;* and the accomplished 
conchologist is enabled, by certain characters, to determine 
the nature of the animals which inhabited them, and the 
physical conditions in which they were placed. 
The number of living species of mollusca known to natu- 
ralists, not including the shell-less genera, exceeds twelve 
* In equivalve bivalves the animal lives in an upright position. In 
inequivalves, z.¢, one large and one small valve, the animal lies on its 
side. The situation of bivalve shells, as oysters, should therefore be 
noticed, for if they lie on their concave shell, with the flat valve 
uppermost, it is evident they were overwhelmed in their native bed 
and in a living state; if they lie indiscriminately on either valve, they 
were probably dead shells and overwhelmed in that state. If the 
pallial imprint is notched by a sinus, it shows the presence and size 
of the tubes of the mantle. Whether there be one or two muscular 
impressions is of far less importance. 
+ For an extended notice of the geographical distribution of 
testacea, see Prof. Edward Forbes, British Marine Zoology, Part I. 
p. 141. 
