LEPIDOSIREN PARADOX A. 
1)1 
the sinu-auricular opening (Text-fig. 13^ r. S. ^1.). Dorsally 
this fold is lost on the auricular roof^ while ventrally it 
curves round on to the right side of the auriculo-ventricular 
ridge. The development of this right sinu-auricular fold is 
therefore identical with that of the similar right venous 
valve in Elasmohranchs (21). 
The auriculo-ventricular aperture is defined by the marked 
localised bulgings of the auricle aud ventricle on either side 
of the extremely short auricular canal. The auricles expaud 
markedly over the dorsal curvature of the hearty while tlie 
compaiatively much slighter degree of ventricular expansion 
occurs wholly along the ventral curvature (Text-fig. 9 b). 
This disproportion of growth results in a marked constric- 
tion externally, that is manifested in the interior of the 
heart by the formation of an acute auriculo-ventriculai- 
angle or ledge, that curves round but does not quite 
encircle this aperture. The ledge is most prominent where 
the expanding auricle brdges over the first part of the dorsal 
curvature, that is, over the dorsal wall of the proximal part 
of the bulbus, thus forming the bnlbo-auricular groove (Text- 
fig. 9 B and PI. 5, fig. 1, B. Ag.). On the posterior wall 
this constriction and its corresponding ledge reach the sides 
of the auriculo-ventricular ridge (plug), and, when the 
cardiac musculature appears (Stage 30), muscular bundles — 
musculature of the auricular canal — grow round this ledge 
and into the ridge posteriorly from either side, thus becoming 
continuous with the musculature of the developing inter- 
ventricular septum. As development proceeds, the auriculo- 
ventricular opening becomes somewhat horseshoe shaped 
with the convexity anterior. 
The bulbus also comes to be distinctly separated from the 
ventricle, partly by the disproportion of their rates of expan- 
sion, aud partly by the increasingly abrupt curve with which 
the heart is bent on itself in this region. No valvular struc- 
ture, apart from the spiral fold, appears in the aperture 
between the bulbus and the ventricle during any period of 
development. The bulbo-ventricnlar opening is, from the 
