GINGLYMACONCHBA. Gas 
The mantle is generally folded twice, and covers the body ; 
its lobes frequently unite anteriorly, but are sometimes open for 
the exit of the foot. 
They are all furnished with two shells or valves, the hinge 
of which is furnished with an internal or external one hinge- 
shaped ligament which causes the valves to open, even when 
the animal is removed from it; the valves are closed by one or 
two muscles and two elastic tendons ; there is likewise a muscle 
on each of the valves posteriorly for withdrawing the respira- 
tory tubes; in the Pinnadz the abdomen and the foot are sup- 
ported by two longitudinal muscles, which arise near the base 
of the posterior abductor muscles (which seem to be two) ; unite 
before they join the foot, sending out posteriorly and obliquely 
on each side six or seven distinct branches; and after again 
passing the foot, they separate and are inserted near the base of 
the anterior adductor muscles (see Pl. IV. fig. 4): these mus- 
cles form peculiar impressions not found in any other of the 
Families. 
The brain is situated over the mouth, and is composed of 
two or three ganglions. 
The branchize are almost always composed of two elongated 
foliations or rather lamellee, beautifully reticulated with the 
venous and arterial vessels, between which the water passes 
for the purpose of respiration: from these branchize the blood 
returns to the heart, which is generally a mere ventricle which 
sends the blood to every part of the body; it returns by a 
strong pulmonary artery which performs the functions of an- 
other ventricle. 
All these animals fecundate themselves ; the young ones are 
innumerable: they remain for a certain time in cells above the 
upper junctions of the mantle (which serves the functions of a 
uterus and oviduct combined), until the animals are fit to pass 
out of it into the world. 
They are all aquatic. 
All the animals forming this class are generally known by 
the names of bivalve shells, from having their bodies protected 
by a shell on each side of the body; these valves are most 
generally connected together at their hinge by an elastic liga- 
