98 
JANE I. EOBEIITSON. 
septum (Text-fig 15 a, b, c, d, av. r.), partly to the compara- 
tively late development of the pulmonary vein — is maintained 
throughout, though in the adult the disparity is very slight. 
The interventricular septum in Lepidosiren would thus 
appear to be homologous with the incomplete posterior ven- 
tricular septum of Lacerta, and therefore also with the 
posterior muscular part of the complete interventricular 
septum of the Alligator or Crocodile ( 9 ). 
Musculature of the Heart. — From the first the dense 
musculature and comparatively rigid walls of the auricular 
canal and proximal part of the bulbus cordis form a marked 
contrast to the rapid peripheral expansion and loose tra- 
becular meshwork that characterise the ventricular and 
auricular chambers of the heart. As the ventricle expands 
its comparatively thin walls bulge round the compara- 
tively rigid auricular canal and bulbus cordis respectively. 
The result at the auriculo-ventricular aperture is that the 
auricular canal has the appearance of being invaginated 
into the ventricle ; really the ventricle has grown up round 
it. This brings the musculature of the auricular canal 
into direct relation with the posterior trabeculse of the 
developing interventricular septum postei-iorly, and laterally 
with the endocardial surface of the ventricular musculature 
round the periphery of the auriculo-ventricular opening 
(Text-fig. 18, A, C). Anteriorly, the auricular canal comes 
directly in contact with the posterior wall of the proximal 
part of the bulbus at the bulbo- auricular fold (Text-fig. 
9 B, B. Ag.) and the musculature of these two parts is 
continuous round it. On the auricular side the musculature 
of the auricular canal tapers off imperceptibly into that of 
the auricles (Text-fig. 17, A. G.). At the bulbo-ventri- 
cular aperture the ventricle bulges laterally and anteriorly 
round the proximal part of the bulbus, so that it also projects 
somewhat into the ventricle, but here the process appears to 
be more strictly a folding between the two divisions of 
the heart, and the ventricular and bulbar musculatures pass 
into one another round the edge of the fold. The muscula- 
