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JANE I. ROBERTSON. 
calar orifice (PJ. 5, fig. 1) ; tlie left compartment receives 
in front the left ductus Cuvieri. 
The sinu-auricular aperture (PI. 5, fig. 1) is compara- 
tively large and oval in shape and opens into the right 
auricle immediately to the right of the pulmonary fold (PI. 
5, fig. 1,^9. /.). The opening is guarded on its right side 
by the vertical fold already mentioned that projects from tlie 
region of the sinu-auricular groove (PI. 5, fig. 1, r. 8. A.) ; the 
degree of development of this fold, however, varies very 
much in different specimens ; in one it was represented merely 
by irregular thickened projections of the right rim of the 
sinu-auricular opening. A similar variability is noted for 
Protopterus (2), Amia, Lepidosiren and Polypterus 
( 21 ). 
Auricles. — The auricles are large, extremely thin-walled 
structures which, when dilated, bulge round thebulbus cordis 
and ventricles so as almost completely to surround them ; 
they are markedly lobed and their margins are more or less 
digitate. There is no distinct external division between the 
two auricles, the situation of the interauricular septum being 
indicated only by a faint groove. The right auricle is the 
larger of the two ; its long anterior process is wrapped round 
the ventral surface of the distal part of the bulbus cordis, a 
middle or transverse process reaches the middle line of the 
ventral surface of the heart at the anterior end of the 
ventricles, and its lower margin reaches, in some instances, 
to the apex. Both auricles are attached round the margins of 
the auriculo-ventricular aperture (PI. 5, figs. 1 and 2), and are 
held applied against the bulbus by the lymphatic sheathing 
already described. The ventral wall of the auricle and the 
dorsal wall of the bulbus cordis form a sharp projecting rim 
internally, and a corresponding bulbo-auricular groove exter- 
nally round the anterior part of the auriculo-ventricular 
opening (PI. 5, figs. 1 and 2, B.A. g.) Dorsally and to the 
right the auricles are attached to the sinus venosus, but 
only the right auricle communicates with that compartment. 
Posteriorly, projecting into and between the auricles where 
