LEPIDOSIREN PAEADOXA. 
65 
attached along the left wall somewhat dorsally (PI. 5, figs. 
2 and 4, B. R. 3.) ; this unites for a short distance at its distal 
end with the spiral valve, dividing the cavity of the ventral 
aorta into a dorsal and ventral passage; this short partition 
wall fuses at its distal extremity with the dorsal wall of the 
ventral aorta in front of the region of the fifth and sixth 
aoi tic arches, terminating in a little cushion-like projection 
(PI. 5, figs. 2 and 4, S. Ao.). The result is the formation of 
a ventral passage communicating with the two anterior, and a 
dorsal passage communicating with the two posterior, pairs 
of aortic arches. 
Coronary Arteries. — Two little arteries are present one 
on each side of the bulbus cordis. The right vessel is the 
larger; it passes along the right wall of the distal part of the 
bulbus, dorsal to the transverse part, and then along the left 
wall of the proximal part, to the ventricle, where it is 
distributed. The smaller left artery supplies the left dorsal 
wall of the distal part of the bulbus. The course of these 
little vessels anterior to their appearance on the sides of the 
bulbus has not been traced, owing to the difficulty of 
dissecting the fibrous tissue in the region of the ventral 
aorta. 
Summary. — The main features of importance in the 
anatomy of the heart of Lepidosiren may now be sum- 
marised, and some suggestions put forward as to their probable 
physiological significance. 
In Lepidosiren, as we have seen, auricle, ventricle and 
bulbus cordis are each more or less incompletely divided into 
two right and left chambers, and as the septa of all these 
compartments (the proximal part only of the bulbus being 
considered for the moment) are approximately in the same 
plane, it follows, therefore, that the three chambers on the 
right and the three on the left are in sequence respectively. 
The sinus venosus opens into the right auricle, and its 
aperture is protected on the right side by a more or less 
efficient valve (PI. 5, fig. 1, r. S. A.). The pulmonary vein 
opens directly into the left auricle amd is also protected by a 
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