56 
JANE I. ROBERTSON. 
while the auriculo-ventricular plog or fibro-cartilage is 
apparently better adapted for closing the auriculo-ventricular 
opening than the similar structure in the heart of Cera- 
todus. In short, the heart of Lepidosiren, as compared 
with that of Ceratodus, shows a marked advance in the 
development of a mechanism for the more complete sepa- 
ration of the arterial and venous blood-streams. 
The general facts of the development of the heart are 
comparable with those described for the Elasmobranchs (12) 
in so far as the division into different chambers is concerned ; 
the details, however, differ considerably. In Lepidosiren 
the auricular canal is only transitorily a distinct external 
division of the heart, and early loses its identity in the 
auriculo-ventricular opening. The posterior auricular wall 
remains very short and the sinu-auricular and auriculo-ventri- 
cular openings are consequently close together. The auriculo- 
ventricular ping, which is apparently a peculiarly dipnoan 
structure and which seems to be identically developed in 
Ceratodus (14), can, however, be compared in origin and 
function both to the posterior (dorsal) auriculo-ventricular 
valve of the Elasmobranchs, and to the posterior (dorsal) 
sinu-auricular valve in Lepidosteus (2), but at no period is 
there any equivalent to an anterior (ventral) valve. The right 
sinu-auricular valve develops as the similar structure in the 
Elasmobranchs, but it is often very poorly developed and 
there is no left valve. The pulmonary vein develops in the 
left wall and roof of the sinus venosus, and opens, fi-om the 
first, directly into the left auricle as in the reptilian heart (21), 
the auricular connection of this vein in the Urodele being a 
secondary, not a primary condition. The interauricular 
septum of Lepidosiren develops in the same way as that 
of the Urodeles (12)^ except that the termination of the pul- 
monary vein projects somewhat into the auricles in the 
former, and actually forms the most posterior part of the 
septum. The development of muscular trabeculae in the 
ventricle proceeds as in all the Yertebrata; similarly they 
extend into the auricular canal and become continuous with 
