Prof. OwxN's Description of the Lepidosiren annectens. 345 
does not, however, communicate with the sinus, but passes along entire and 
adherent to the inner surface of the vena cava as far as the auriculo-ventricular 
aperture, where it empties its contents into the ventricle by a distinct orifice, 
protected by a cartilaginous valvular tubercle. It needed only that the pul- 
monary vein should have been dilated before its termination in order to have 
established a biauricular structure of the heart, as in the Siren. The same 
functional advantage is, however, thus secured to the Lepidosiren, with a 
maintenance of the simple dicælous type of the heart of the Fish: the conti- 
nuation of the pulmonary vein preventing the admixture of the respired with 
the venous blood, until both have arrived in the ventricle. 
The ventricle* is of an elongated form, truncate anteriorly where it is in 
contact with the bulbus arteriosus, and with an obtuse rounded apex at the 
opposite end: it is four lines in length, and two in breadth. The cavity of 
the ventricle is extremely small; its parietes are thick and reticularly muscu- 
lar: a small round orifice leads into the bulbus arteriosus, This body t pre- 
sents externally a simple transversely oval form; bnt its internal structure 
is more complicated than would be suspected from its external appearance. 
It is formed by a short spiral turn of the dilated aorta, which is concealed 
under a simple continuous outer fibrous coat: the area of this part of the ves- 
sel is almost entirely occupied by two continuous valvular projections, or their 
processes, which are attached by one edge to the internal surface of the 
aorta, and have the opposite margin projecting freely into the arterial cavity. 
If these internal valves were straight, they would resemble the single thicker 
valvular process which occupies the elongated bulbus arteriosus of the Siren : 
here, however, they follow the spiral turn of the aorta. 
The aortat in the present most remarkable species fulfils at once the office 
of a systemic, a branchial, and a pulmonary artery : it distributes on each side 
six vessels corresponding to the six branchial cartilaginous arches. The 
mucous membrane is produced into a branchial fringe on the convex side of 
the Ist, 4th, 5th, and 6th branchial arches, and the corresponding arteries are 
minutely subdivided before they are continued to the dorsal side of the pha- 
rynx: these four pairs of vessels are therefore true or functional branchial 
arteries. The mucous membrane merely invests with a simple fold the second 
* Tas. XXVI. fig. 2, ö. T Ib. fig. 2, c. 1 Ib. fig. 2, g. 
2z2 
