BIVAL VES. 
401 
and in Schizodentalium a series of holes takes the place of a ventral slit. Fossil 
tooth-shells are numerous from the Devonian epoch upwards, but are most abundant 
in the Tertiaries. The living forms, of which about a 
hundred have been described, occur in all parts of the globe, 
and have been dredged at the greatest depths, although 
they are probably most abundant in a few fathoms. 
The Bivalves, —Class Pelecypoda. 
This great division includes all forms which secrete a 
bivalved shell, like the oyster, cockle, and mussel. The 
Pelecypods offer a different type of organisation from that 
prevailing in the other classes. The absence of a head, and 
the bilateral symmetry of the animal, enclosed within a 
bivalved shell, are characteristic of the class. The mantle 
is divided into two similar lobes (r/), right and left, forming 
a flap on either side of the body, to which it is connected 
at the upper part beneath the hinge-line of the shell. It is 
usually very thin, excepting at the edges, which are some¬ 
times double or even threefold. In some genera the edges 
of the two lobes are free or unconnected at any point a > F 2indS ai mlnt C i a e V c t a vity • 
excepting at the dorsal attachment, in others they are joined 
in one or more places, leaving orifices for the protrusion of 
the foot (a) in front, and for the entrance of the water to 
the gills ( d, e ), and for the extrusion of waste and other 
matter at the posterior end (/). The mantle at this posterior opening is often 
considerably produced, forming one or two 
distinct tubes or siphons (A), which vary 
considerably in length in different groups, 
equalling in some instances several times 
the length of the shell. The extreme 
development of these siphons obtains in 
Teredo, where they constitute the principal 
mass of the animal. Some pelecypods 
appear to be all foot, this member being 
enormously developed in the razor-shells 
(Solen), for example. In others it assumes 
smaller dimensions, or it may be absent, 
as in the oyster. It is used either as a 
means of locomotion, or for burrowing in 
sand or mud, or perforating rocks, wood, 
and other substances; its form conse¬ 
quently being very variable. The mouth 
(b) is situated at the anterior end of the 
body, at the upper front part of the foot, 
soft-parts of river-mussel ( Anodonta ). forming a simple transverse aperture. The 
VOL. vi.—26 
SECTION OF ANIMAL OF 
Dentalium (enlarged). 
b, Mouth process; c, 
Anal opening; d, Foot- 
cavity ; e, Tentacle sup¬ 
ports ; /, Posterior end. 
-.9 
