58 
JANE I. ROBERTSON. 
cava develops centripetally towards the hearty and is to be 
looked on as a short cut connecting the large right posterior 
cardinal vein with the heart. Both posterior cardinals 
persist in the adult, the left in its entirety, the right only as 
a comparatively short vessel. The development ot the sub- 
intestinal and portal veins is essentially Elasmobranch (20) in 
character. 
The vascular system of Lepido siren would thus appear 
to liave many points in common with both the Elasmobranchs 
and the amphibians, and to occupy a position between the two, 
thouo'h tendinsc more towards the latter. As was to be 
expected, the adult conditions are almost identical with those 
described for Protopterus; some of the main points agree 
with those given for Ceratodus, but the details of the 
vascular system of the latter — apart from the heart — appear 
to be much more closely piscine than in Lepidosiren. 
The Heart and Vascular System of the Adult Lepidosiren. 
(a) Heart. 
In Lepidosiren the heart is placed far forward and lies in 
a thick pericardiac envelope that splits into two layers in the 
lateral and ventral portions of its posterior half. This split 
forms a lymph space over the posterior part of the ventral 
surface of the inner pericardium ; the two pericardiac la^mrs 
come together again at the entrance of the posterior vena 
cava into the sinus venosus. The pericardium is attached to 
the heart over the dorsal surface of the sinus venosus, round 
the posterior vena cava as it enters the sinus, to the ventral 
surface of the ventricle by a special fibrous band — gubern- 
aculum cordis of Fritsch (7) — and to the anterior extremity 
of the bulbus cordis. The outer surface of the pericardium 
is firmly attached at the sides to the body musculature and 
anteriorly to the pectoral girdle. 
The heart as a whole is of irregular oval shape (PI. 5, figs. 
