L E P I DO S I R EN P AlUDOX A . 
57 
the valvular apparatus, the auriculo-veutricular plug, situated 
there. This structure, however, with its thick, centrally 
placed muscular attachment on its ventral surface, does not 
become hollowed out to form a pocket valve, but gives rise 
to the typically dipnoan auriculo-ventricular plug. The de- 
velopment of the long bulbus cordis resembles in some degree 
that of the same division of the heart in the Elasmobranchs, 
e.g. in the appearance fora time during development of four 
endocardial cushions in the distal and proximal parts of the 
bulbus, and the persistence proxiiually in the adult of three 
rows of vestigial pocket valves, as well as of transverse 
furrows at the base of the spiral valve. The discontinuous 
development of the spiral valve in two segments can be com- 
pared with the similar condition in Urodeles (16). The heart 
of Lepidosiren therefore presents different elasmobranch, 
amphibian, and even reptilian, as well as some peculiarly 
dipnoan characters. 
Some of the main points of interest in the arterial system 
are the development of six aortic arches, and the close 
resemblance of their ultimate arrangement to that present in 
the Urodeles (3), the lingual and dorsal carotid arteries of 
Lepidosiren corresponding to the external and inteinal 
carotids of the former. The pulmonary arteries are developed 
directly in connection with the sixth aortic arches, and come 
to open secondarily from the dorsal aortic roots; whereas in 
the urodele they are tending to become more direct continua- 
tions of the sixth aortic vessels. The development of the 
coeliac artery as primarily the right glomerular vessel is com- 
parable with the teleostean condition, but its lack of further 
connections with any other vessels except in the wall of the 
gut appears to be a detail peculiar to Lepidosiren. 
In the venous system the development of the anterior 
cardinal veins tallies generally with that of the Elasmobranchs 
and amphibians, while that of the posterior cardinal veins 
agrees with the amphibian type, except that in Lepidosiren 
the fused interrenal portions of the two vessels become 
separated again. The anterior section of the posterior vena 
