24 BRITISH MARINE TESTACEOUS MOLLUSCA : 
their junction with the body, under the mantle, at the dorsal 
range, and is of concurrent length with them; each branchial 
vein coasts the margins of its respective lamina. The motive 
power of the circulation is a heart, which is a simple ventricle, 
and two symmetrical, oblong auricles, one on each side of it; 
it is placed subcentrally on the dorsal range, rather nearer to 
the posterior half, and with the auricles embraces the rectum 
or intestine, which thus apparently passes through it. But 
this is not the case with every bivalve, though the major part 
of them are thus constituted; the Ostree, Arce, and some 
few others are exceptions. The heart and auricles are nearly 
pellucid, of the thinnest texture, and apparently of feeble 
power; the auricles have valves to prevent a reflux of the 
blood; they are usually situate at their junction with the 
heart, sometimes at the pomt where the branchial veins unite 
with them, and they sometimes are found at both contacts. 
The inspirations and expirations are each about seven or 
eight a minute. The heart gives forth at least two aortic 
branches, which ramifying furnish arteries to the system, and 
at their invisible termini the venous reflux has its origin, and 
by continual anastomoses and imosculations, in their progress 
to the arterial centre, terminate in one or more trunk veins 
or ven cave, which effect a coalition with the branchial 
artery. 
Though the circulation is what is called complete, it may 
be lable to interruptions, divergences and retardations ; that 
is, if the opimion of some zoologists, Mr. Garner amongst them, 
is well founded, that the whole of the blood is not returned 
to the body in each circulation, and a part of it, from various 
organs, passes to a set of ves which are concentrated ante- 
riorly, on each side the dorsal range, in an elongated sinus, 
extending to and under the pericardium ; and that these veins, 
by small lateral internal sacs, ducts or orifices, pass the secre- 
tions, or fluids they have distilled, to the mantle, in aid of 
structure and coloration, for the use of the ova and other 
organs; and that branches from this fasciculus of veins, after 
the depuration of their contents, transmit the blood into the 
branchial artery; it is also said that other portions of the 
