LEPIDOSIREN PARADOXA. 
61 
they are attached to the posterior margin of the auriculo- 
ventricular opening, is a fold in which the pulmonary vein 
reaches the left auricle (PI. 5, fig. 1,^?./.) ; this fold projects 
across, and is attached along the dorsal surface of, the 
auriculo-ventricular plug and forms the posterior part of the 
iuterauricular septum. The cavities of both auricles are 
traversed from roof to floor by a delicate loose mesh work of 
muscular bands, except in tlie region of the auriculo-ven- 
tricular aperture ; this meshwork condenses in front of the 
pulmonary fold and is attached to it to complete the inter- 
auricular septum (PI. 5, figs. 1 and 2, A.S. and P.f.). The 
meshwork passes from the anterior margin of the pulmonary 
fold to the ventral auricular wall (applied against the dorsal 
wall of the proximal part of the bulbus), arching across the 
narrow space between the margin of the auriculo-ventricular 
plug and the anterior rim of the auriculo-ventricular opening 
(PI. 5, figs. 1 and 2). 
The closeness and extent of this trabecular meshwork in 
the region of the auricular septum seems, however, to vary 
greatly indifferent specimens: in some the trabeculae are 
numerous and compactly arranged, projecting considerably 
into the auricle, and there may be sheets of fine connective 
tissue between them, while in the others the trabeculae are 
fewer and widely separate without any intervening connective 
tissue. 
The iuterauricular septum, therefore, is composed of two 
elements which are joined together : a pulmonary fold pos- 
teriorly, and the auricular meshwork anteriorly (PI. 5, figs. 1 
and 2). This septum, however, does not ever completely shut 
off the two auricles from one another, there being always a 
space left just over the auriculo-ventricular opening. 
As already noted, the pulmonary vein opens into the left 
auricle on the left surface of the posterior part of the inter- 
auricular septum (pulmonary fold), immediately dorsal to the 
left half of the auriculo-ventricular plug, and its opening is 
guarded by a hood-like fold that is attached ventrally to the 
anterior rim of the auriculo-ventricular plug (PI. 5, fig. 2, 
