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JANE I. IIOBEKTSON. 
closing the auriculo-ventriculai’ apertui’e. (Compare Text- 
figs. 17 and 18, av. r.) 
Finally, the cellular matrix of which this structure is 
composed now develops into the cartilaginous tissue that 
gives it its characteristic consistency. 
This auriculo-veutricular plug, therefore, originates in a 
little cellular proliferation at the left sinu-auricular margin. 
It extends along the short posterior auricular wall and lies in 
the auricular canal, projecting a little into the ventricular 
part of the heart. It is intimately related to the interven- 
tricular and interauricular septa, as well as to the pulmonary 
fold; throughout it is in proximity to the sinu-auricular 
opening and itself occupies the auriculo-ventricular aperture, 
while its actual area of attachment to the heart wall is the 
only representative in the adult Lepidosiren of the pos- 
terior auricular wall of the embryonic heart. The origin and 
situation of this plug would point to its homology with the 
posterior (dorsal) pocket valve of the auricular canal in elas- 
mobranchs ; in Lepidosiren, however, there is at no time 
any trace of an anterior (ventral) valve. Again, its relations 
to the posterior (dorsal) arch of the interauricular septum 
and to the opening of the pulmonary vein point to its 
homology also with the posterior (dorsal) auriculo-ventricular 
endocardial cushion of the urodele ( 12 ). Thus the auriculo- 
ventricular plug in Lepidosiren may be regarded as a 
modified auriculo-ventricular pocket valve. Boas ( 2 ) suggests 
that the auriculo-ventricular plug of Ceratodus, which is a 
structure similar in position and general relations to that of 
Lepidosiren, arises from the fusion of the posterior 
(dorsal) valves of the approximated sinu-auricular and 
auriculo-ventricular openings, and compares the heart of 
Ceratodus with that of Lepidosteus, where these two 
sets of valves are comparatively wide apart and quite 
distinct. As has been shown, the development of the 
auriculo-ventricular ridge in Lepidosiren distinctly sup- 
ports this suggestion. The ridge appears first at the 
sinu-auricular junction and extends uninterruptedly along 
