LEPrDOSLItl^]^ PARVDOXA. 
99 
tui '0 of the seeDiid and third parts of the bulbus is very 
poorly developed. 
The relations in Lepidosiren of the developing ven- 
tricular musculature to the short auricular canal and the 
auriculo-ventricular ridge (plug) situated in that canal, 
resemble closely those of the ventricular musculature to the 
posterior (dorsal) auriculo-ventricular pocket valve in the 
^lasmobranch (12). In Lepidosiren, however, the auriculo- 
ventricular ridge does not become hollowed out into a typical 
Text-fig. 18. 
P.V 
Horizontal section through the heart at Stage 32. A. G. Muscu- 
lature of auricular canal continuous with endocardial surface 
of the ventricle. ay. r. Auriculo-ventricular ridge (plug). 
L.A. Left auricle. L.V. Left ventricle. P.V. Pulmonary 
vein. R. A. flight auricle. B. V. Right ventricle. V. S. 
Interventricular septum. 
pocket valve, and its muscular connections with the ven- 
tricular wall form the septum of that compartment of the 
heart. 
Auriculo-ventricular Plug. — With the appearance of 
muscular fibres in the ventricle, the development of the 
interventricular septum and the definition of the auriculo- 
ventricular aperture, the auriculo-ventricular ridge becomes 
greatl}" thickened in its portion immediately dorsal to the 
auriculo-ventricular opening till it assumes the button shape 
that distinguishes its final form and adapts it admirably for 
