lEPlDOSlREN PAllADOXA. 
103 
At the junction of the lateral ventral aortae to form the 
extremely short ventral aorta, the free edges of the ridges 
on their ventral — posterior in the adult — walls (Text-fig. 19, r.) 
fuse in the middle line : the horizontal partition so formed 
unites at its distal extremity with the dorsal wall of the aorta, 
thus cutting off the fifth and sixth from the fourth and third 
pairs of afferent aortic vessels (Text-fig. 21, B. B. 1, B, B. 3). 
Meanwhile the rotation and asymmetrical bulging of the 
bulbus cordis already described have taken place, with the 
result that the consequent kinking towards the left between 
its proximal and middle (transverse) segments brings the 
projecting free margin of the proximal extremity of the 1 ridge. 
Text-fig. 21. 
Transverse section at Stage 36 through the distal end of the 
bulbus cordis (ventral) aorta showing fusion of the right and 
left ridges or valves. B. R. 1. Right ridge. B. B. 3. Left 
ridge. 
and the distal extremity of the left side of the A ridge, into 
close proximity round the rim of the kink between these two 
segments (Text-figs. 12 a, b, and c, and 20 b, B. B. 1, and 
B. B. A.) of the bulbus. 
Consequently the two ridges fuse and form the spiral 
valve. This spiral valve therefore arises in Lepidosiren 
discontiuuously in two portions, one, namely, belonging to the 
ventral wall of the proximal segment of the bulbus (A ridge) 
and one belonging originally to the right wall (B.B. 1 ridge) 
of the distal and middle segments. It has already been 
shown, however, how, in the process of development, the right 
wall of the middle segment was carried transversely and 
somewhat posteriorly and with it the valve situated on its 
endothelial surface. The two portions of the spiral valve. 
