18/4.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 201 



to be the case in Proteus anguinus. On the right side, a vein from 

 the stomach opens into the pulmonary vein." 



Van der Hoeven adds to this description a more precise account 

 of the truncus arteriosus (which be terms arterial trunk, "slagader- 

 lijken stam"), and its terminal dilatation into an elongated oval 

 bulbus arteriosus {I. c. p. 30) :— " At the origin of the truncus 

 arteriosus lie three semilunar valves, and higher up beneath the 

 bulbus three more. In the bulbus itself is a freely projecting solid 

 plate, by which its cavity is almost divided into two semicanals." 



He further states thatthe auricle " is divided by an imperfect sep- 

 tum;" but he gives no account of the form or structure of this 

 septum. 



In the specimen dissected by me, the heart (Plate XXXII. figs. 1 

 to 6) was lodged in an oval pericardial cavity, situated between the 

 styliform secoud basibranqhial and the curious chondrification of the 

 linea alba and of the adjacent parts of the intermuscular septa, 

 which Mayer (I. c. p. So) justly interpreted as the sternum. Thick 

 masses of 'longitudinal muscular fibres lie on each side of the peri- 

 cardium, and represent the sterno-hyoid muscles. 



The heart consists of a dorsal division, composed of the sinus venosus 

 (S) and of the auricles (A), and of a ventral division, consisting of the 

 ventricle (F) and the truncus arteriosus (T. a). The sinus venosuslies 

 immediately over the posterior half of the ventricle, and is formed by 

 the junction of the two inferior caval trunks described by Mayer. 



The right and left superior cavae (R.s.v.c, L.s.v.c.) open into these 

 at their passage into the sinus venosus, and might fairly be said 

 to communicate directly with the latter. The siuu-auricular aper- 

 ture is situated in the right half of the posterior wall of the spacious 

 auricular cavity. Though its lips project somewhat into the auricular 

 cavity, they can hardly be regarded as truly valvular. 



The auricular chamber is very spacious, and extends forwards nearly 

 as far as the anterior end of the pericardium. It lies above the 

 truncus arteriosus in front, and the anterior half of the ventricle 

 behind. On each side, it is produced into a saccular dilatation, which 

 extends over the truncus and ventricle to the ventral wall of the peri- 

 cardium on the left side, but is much less developed on the right 

 side, where it leaves the ventricle uncovered. The wall of the right 

 dilatation presents three or four longitudinal folds. 



On the dorsal face of the sinus venosus, between the two superior 

 cavse and the diverging inferior caval trunks, lies the pulmonary 

 vein (P. v. fig. 4). It is very narrow posteriorly, but dilates in front, 

 and, turning to the left, opens into the posterior part of the auri- 

 cular cavity, to the left of the middle line. 



The existence of the anomalous arrangement of the pulmonary 

 veins in the specimen described by Mayer is therefore rendered 

 doubtful, though I am unwilling to suggest that so accurate an 

 observer was altogether mistaken. 



The right wall of the dilatation of the pulmonary vein is continued 

 downwards, forwards, and to the left side, nearly as far as the dorsal 

 lip of the auriculo- ventricular apeiture, as a delicate plate formed 

 Proc. Zool. Soc— 1874, No. XIV. 14 



