LEl’lDOSIREN PAUADOXA. 
55 
material and for much helpful criticism throughout the course 
of my work. 
The expenses of illustration have been largely defrayed by 
a special grant from the Carnegie Trustees, and this has been 
an assistance that I have much appreciated. 
II. Summary of General Features op the Vascular 
System of Lepidosiren. 
The main points of interest in the comparative anatomy of 
the heart and vascular system of Lepidosiren are here 
briefly summarised. Further references will be found in the 
text in the course of the article itself. 
The heart of Lepidosiren is distinguished by the presence 
of well-marked, though incomplete, auricular, ventricular and 
bulbar septa, the absence of any pocket or semilunar valves, 
and the separate opening of the pulmonary vein into the left 
auricle. Two other distinctive features are the long, abruptly 
curved bulbus cordis, and the single button-like plug that 
acts as an auriculo-ventricular valve. The heart of Lepi- 
dosiren most closely resembles that of Protopterus ( 2 ), 
but differs from it in possessing definite, though incomplete, 
interauricular and interventricular septa. Compared with the 
heart of Ceratodus (2) and (I'i), the main points of difference 
are: (1) The more abrupt kinking of the bulbus cordis; 
(2) the almost complete disappearance in Lepidosiren of 
all the bulbar pocket valves as such, except proximally, 
where a few vestigial valves still persist ( 17 ) ; (3) the develop- 
ment of the spiral valve as a structure that extends through- 
out the length of the bulbus; (4) the presence of a longi- 
tudinal valve on the left wall of the distal part of the bulbus; 
(5) the shortening of the posterior auricular wall, so that in 
Lepidosiren the sinu-auricular and auriculo-ventricular 
openings are closely approximated. In Lepidosiren the 
ventricle is comparatively a larger part of the heart, and the 
interventricular septum is much more definitely a septal 
structure, though its attachments appear to be identical ; 
