BIVALVIA. 5 
far beyond the preceding, that the new border composed of the outer layer, is simply 
joined on to the margin of the former one, so that the successive formations of the 
outer layer scarcely underlie each other. But when the animal has arrived at its full 
growth, the new laminz cease to project beyond the old, and as each is composed of 
a marginal band of the external substance attached to the edge of an entire lamina of 
the inner, these bands must now underlie each other, being either quite free as in 
Ostrea, or closely united to each other as in Unio, and most other Bivalves; and the 
additions to the shells of the Gasteropoda are made upon the same plan, although it 
has commonly been supposed that they are only attached to the edge of the old shell, 
instead of being continued over its entire surface.” 
The figure and size of the foot materially influences the form of the anterior part 
of the shell, while the posterior depends upon the modification of the siphons. 
The degree of development of the nervous system is said to be very variable in 
these animals, and the organs of sense dependant thereon variously distributed, im- 
perfect organs of sight are present in some species, and rudimentary organs of hearing 
have been detected in others, and are possibly present in all: while in some, the sexes 
are separate and distinct, in others they are united or hermaphrodite, microscopic 
animals and plants constitute their principal food. 
Species of this class have been found in the seas of every clime, and inhabit the 
waters of all depths, some few are left dry by the retiring tide, while others frequent 
the bottom of seas, to the depth of 200 fathoms; and the vertical range of many 
species is so extensive, as to render doubtful the allocation of strata from the presence 
of a few fossil forms, with whose habits we are but indifferently acquainted; more- 
over, the habits of all recent species are not, perhaps, necessarily the same as those 
of their prototypes that lived in times long past, and probably, under different 
conditions. 
The authors of the beautiful work upon the ‘British Mollusca,’ now in the course of 
publication, have given many interesting details respecting the range in depth at 
which most of these animals have been obtained, and occasionally, the nature of the 
ground they had selected for their habitation; the generality of species prefer clear 
water and a sandy bottom, but others are frequenters of mud. The bottoms of the 
Crag Seas, judging from the deposits now remaining upon the Eastern Coasts of 
England, appear to have been principally of sand or gravel, with comminuted frag- 
ments of shells; that of the Coralline Crag Sea being generally fine in its particles, 
formed at a depth varying, perhaps, from 20 to as much as 40 fathoms, if the habits of 
the then existing animals were the same as their homologues of the present day. The 
Red Crag Sea appears to have been subject to greater agitation, and was probably 
less in depth, while much of its bottom was of a gravelly character, or of coarser sand. 
The deposits of the Mammaliferous Crag Period present us with characters rather 
more variable ; that which is found near Norwich, being what is called Fluvio-marine, 
formed probably, in a shallow estuary, and composed of sand, gravel, and shells, 
