Prof. Owen'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 tlie simple diccelous 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 bodyf pre- 

 sents externally a simple transversely oval form ; but its internal structure 

 is more complicated than would be suspected from its external apj)earance. 

 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 aorta + in the present most remarkable species fulfils at once the office 

 of a systemic, abranchial, 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 1st, 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 



♦ Tab. XXVI. fig. 2, b. \ lb. fig. 2, c. % lb. fig. 2,y. 



2 z 2 



