916 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
valves preventing the reflux of the blood into the auri- 
cles during the ventricular systole. On the floor of the 
pericardium, beneath the ventricle, are the openings of the © 
reno-pericardial canals. On the anterior part of the floor 
are several openings which are the terminal portions of 
systems of tubules forming the paired pericardial glands. 
The tubules are lined with cells containing brownish-red 
concretions, and are distributed over a wedge-shaped area of 
the mantle extending, ventrally, from the most dorsal 
portion.* The ventricle is prolonged backwards into a 
short narrow neck which still contains the rectum. A 
transverse membranous partition, beginning at the dorsal 
surface of this neck, extends backwards and downwards 
across its cavity, embracing the rectum, and ends so that 
its free edge lies near the ventral surface. This must form 
a valve preventing the reflux of blood into the ventricle 
from the posterior part of the body. Behind this valve the 
tube expands forming a ‘‘bulbus arteriosus” (B.a., fig. 30) 
with thin muscular walls. This terminates in two lateral 
branches, the right and left posterior pallial arteries which 
diverge from each other and pass backwards beneath the 
posterior adductor. Here their walls become very ill- 
defined and communicate freely with a system of lacunz 
between the bundles of the muscle. The arteries, which 
are now difficult to trace, reach the mantle margin and 
terminate in the sinuses there and in the walls of the 
siphons. 
Anteriorly the ventricle passes into a single median vessel 
with well defined walls, the anterior aorta (Ao., fig. 30); 
this pierces the posterior wall of the viscero-pedal mass, and 
travels along in the dorsal region of the latter, giving off, 
in its course, small vessels to the digestive gland. Near 
* Grobben, Dr. C. Die Pericardialdriise der Lamellibranchiaten. Arbeit. 
Zool. Inst. Wien. Bd. VII. 1888. 
