14 Cincinnati Society of Natural History. 
receives the blood from the venous system. It is an elon- 
gated thin-walled sac, about 2 cm. in length, situated a little 
to the left of the median line, just above the anterior lobe of 
the liver, and between the sheets of mesentery which form 
the hepatic ligament. The larger posterior portion lies just 
below the alimentary canal, and anterior to the gall cyst. The 
smaller anterior portion lies beneath the auricle (A). The 
sinus lies outside the pericardial cavity, but is partially 
enclosed between the layers of the mesocardium. It is 
dilated posteriorly, being somewhat quadrilateral in outline, 
and compressed laterally. In this region the sinus receives, 
on the right side, the anterior and posterior hepatic veins 
(ha, hp). At the posterior extremity it receives the pos- 
terior common cardinal trunk (pec). Anteriorly, it receives, 
on the left side, the left anterior cardinal vein (acl). The 
narrow anterior portion of the sinus venosus receives the 
inferior jugular vein (jv) just anterior to the sinu-auricular 
aperture, which lies in the dorsal wall near the anterior end. 
The auricle (Figs. 1, II, III, A) lies immediately above and 
anterior tothe sinus. It occupies the left portion of the peri- 
cardial cavity, and is surrounded by the pericardium (its rela- 
tions to which will be described in detail later). The auricle 
is the largest of the cardiac divisions, and, when expanded, 
almost completely fills the left pericardial cavity. When con- 
tracted, however, it is smaller than the ventricle, since it is 
very thin-walled. It is an elongated sac, irregular in shape, 
and flattened dorso-ventrally. ‘The exact position and extent 
varies considerably in different specimens. It is somewhat 
irregularly convex, except the inner and upper walls. These 
are usually concave, on account of being closely apposed to the 
ventricle during life. The auricle usually lies for the most 
part on the left side of the alimentary canal. Posteriorly, the 
auricle is closely connected ventrally with the sinus, com- 
municating with it through the sinu-auricular aperture. On 
the right side it communicates with the ventricle through a 
short canal, the ductus auricularis. (See Figs. landII.) This 
duct, though well marked in some specimens, is obscured in 
others by the approximation of the auricular and ventricular 
walls. It is really a constricted portion of the auricle at the 
auriculo-ventricular aperture. The margin of the auricle 
2 
