BIVALVES. 
4°3 
United along the dorsal margin, but they are never connected on the ventral side. 
Bivalves clothe their shells with a more or less distinct periostracum, which is some¬ 
times thin, smooth, and shining, and often of a yellowish or olivaceous tint, or it 
may be thick, pilose, velvety, or rugged. It has been already noticed that the 
valves are nearly always connected dorsally by a ligament. This is not, however, 
the means by which they are held together, or closed with such force; this closure 
being effected by one or two muscles (adductors), firmly attached to the inner 
surface of the valves, and endowed with such power of contraction, that it is an 
impossibility to force them apart with¬ 
out injury. The places of attachment 
of these muscles (m, m') are generally 
visible, as well as other minor scars 
caused by the pedal retractor muscles. 
The point of attachment of the edge 
of the mantle, known as the pallial 
impression, is often quite distinct; it is 
parallel with the lower margin of the 
valves, and, in some groups, is more or 
less deeply sinuated ( n ) below the 
posterior adductor impression. All 
pelecypods are aquatic, the majority 
being marine. They are less numerous 
than gastropods in .species, but in in¬ 
dividuals are at present, as in past ages, relatively far more so. They are found at 
all depths, from low-water mark; many having been dredged in more than two 
thousand fathoms. Bivalves, however, are most abundant in shallow water. They 
live buried in the sand or mud, or attached to rocks and other substances, either by 
the shells themselves, or by means of a byssus, consisting of horny fibres secreted by 
a gland near the extremity of the foot. Others bore into rocks, wood, and other 
substances, and a few take up their abode in the tests of certain Tunicata, and in 
sponges, in the grooves of sea-urchins; and one species ( Entovalva ) lives parasiti- 
cally inside a sea-cucumber. Dr. Pelseneer divides the class into five orders, based 
mainly upon the structure and morphology of the gills, but at the same time upon 
the general conformation of the animal. To explain in detail the differentiating 
anatomical characteristics of these orders would be beyond the scope of the present 
work, and consequently only salient features can be mentioned. 
Order Protobranchiata. 
In these bivalves the gills have simple unreflexed filaments, disposed in two 
rows in opposite directions; the foot being expanded, with crenulated margins, 
and with scarcely any byssal gland. The families Nucididce and Solenomyidce 
constitute this order. The molluscs belonging to the former are all marine; and 
have the mantle free all round, or forming two small posterior siphons. The gills 
are small, but the labial palpi very large. In the typical NucalIcl the shell is 
small, more or less triangular, generally covered with a greenish olive periostracum 
(/ 
left valve of Meretrix. 
a, Anterior; b, Posterior end; c, Umbo; d, Ventral 
margin ; to', Anterior adductor scar ; to', Posterior : 
n, Pallial sinus. 
