LEPJDOSIREN PARADOXA. 
59 
1 and 2) measuring in one large adulfc male 2^ by 1 in. ; 
the ventricle forms little more than lialf the length, 
the rest being due to the bnlbus cordis. The heart is 
fixed in the pericardiac cavity as described above ; it 
is almost mesial in position, and the apex is directed 
backwards, a little ventrally and to the left. The heart 
is wrapped in a fine sheath of lymphatics, which 
invests the ventricles and auricles closely, but is looser and 
more prominent over the bulbus and in the grooves between 
the different cardiac compartments, the auricles in fact are 
held applied to the posterior and lateral surfaces of the 
bulbus by this sheathing. 
Sinus Venosus. — The sinus venosus is a comparatively 
large, thin-walled compartment of irregular pear shape, 
situated posteriorly on the dorsal surface, and to the right 
side of the heart (PI. 5, figs. 1 and 2, S.V.). It is demarcated 
from the auricles by a groove, especially well marked on the 
right side, which is the external expression of a fold guarding 
the sinu-auricular aperture (PI. 5, fig. 1, r. S. A.) ; the roof or 
dorsal wall of the sinus is continuous with the pericardium 
(PI. 5, figs. 1 and 2, Per.) and the pulmonary vein (PI. 5, 
fig. 1, P. V.) crosses it from right to left; posteriorly, the 
posterior vena cava (PI. 5, fig. 1, P. V. C.) opens into the 
sinus and in front of this the little coronary vein (Pi. 5, 
fig. 1, C. V.) from the dorsal wall of the right ventricle opens 
on its floor. The ventral wall of the sinus rests upon 
the dorsal wall of the ventricles posteriorly and upon 
that of the auricles anteriorly where the two ducts of 
Cuvier open into it (PL 5, fig. 1, r. D. C. and 1. D. C.). 
The apertures of these vessels are separated from one 
another by the posterior border of a fold projecting 
obliquely from the roof and anterior wall of the sinus and 
which contains the pulmonary vein. This oblique fold 
divides the anterior part of the sinus somewhat unequally 
into a smaller left and a larger right compartment : the right 
compartment receives in front the right ductus Cuvieri, whilst 
below this, to the right of the oblique fold, is the siuu-auri- 
