LEPIDOSI RI^]N PARADOXA. 
87 
paratively close to one another. In Lepiclosiren the auricular 
canal is only a transitory division of the heart externally^ 
and early merges its identity in the auriculo-ventricular 
aperture ; in the adult its wall is represented by a little 
flattened muscular band;, that passes round the rim of the 
auriculo-ventricular opening on to the dorsal auricular Avail, 
on which is situated the auriculo-ventricular plug. 
Idle ventral and lateral walls of the transverse part of the 
heart expand rapidly (PI. 5, figs 5 and 6, V.) while the dorsal 
curvature and the distal part of the ventral curvature of the 
heart form the proximal part of the bulbus cordis (compare 
Text-fig 9, A, B and c, B.). Tlie ventricle proper is formed 
by the expansion of the ventral and lateral walls of the 
transverse part of the heart between the auriculo-ventricular 
and ventriculo-bulbar apertures, causing it to bulge laterally 
and a little posteriorly round the auricular canal and laterally 
and anteriorly round the proximal part of the bulbus cordis. 
For a time the ventricular part of the heart is considerably 
tilted to the left, but Avith the swinging of the bulbus to the 
front and the development of the interauricular and 
interventricular septa, this gradually disappears, and in the 
adult the ventricle is approximately symmetrically placed 
about the middle line (PI. 5, figs. 5, 6 and 7, F.). 
A o-eneral increase occurs in the dimensions of the bulbus 
cordis also, but this expansion is most marked in its middle 
part (Text-fig. 9 a, b, and C, B.), affecting particularly the 
anterior and left Avails of this region (PI. 5, figs. 5 and 
6, B.). The distal segment, on Avhose right Avail the right 
septum has appeared reaching along the middle part as far 
as the proximal part of the bulbus, and on Avhose left 
Avail the shorter left septum is present (Text-fig. 9c, 
B. R. 1 and B. R. 3.), undergoes a much lesser degree of 
expansion. The narroAV tubular character of the proximal 
part of the bulbus formed by the dorsal curvature of the 
heart and the distal segment of the ventral curvature has 
already been referred to. 
The bulbus as a Avhole, hoAvever, is noAv rapidly increasing 
