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Venous System of Didelphys Marsupialis 
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pying a position ventral to the abdominal aorta, a position which is 
unusual in mammals. 
Previous to 1893, so far as known to the writer, only the following 
have contributed to the anatomy of the venous system of the marsu- 
pials: Martin, 36, Owen, 35, 36, 39-47 and 66, Forbes, 81, and Cunning- 
ham, 82. From 1893 up to the present time, Hochstetter, 93, Beddard, 
95, Parsons, 96, Windle and Parsons, 98, and the writer, 00, or and 02. 
Of all the above-mentioned investigators Hochstetter was the first to 
give an accurate and comprehensive description of the anatomy of the 
postcaval vein for a large number of marsupials. 
In dealing with the anatomy of the venous system of Didelphys it 
will be the purpose of this paper not to enter into a detailed descrip- 
tion of all the veins, but rather to give an account of the general 
arrangement and principal variations of the caval veins and their chief 
tributaries, in order that the description may serve as a basis for com- 
parison with the veins of other mammals. In addition to this, the 
main features of the heart will be discussed and a general comparison 
drawn between the venous system of Didelphys and that of other mar- 
supials. 
THE HEART. 
The heart of Didelphys presents only a few characters in which it 
differs from that of other mammals, and will be considered under the 
following topics: (1) The fetal structures; (2) the auriculoventricular 
valves; (3) the pulmonary veins, and (4) the coronary veins. 
1. The Fetal Structures.——A fossa ovalis, annulus ovalis and ductus 
arteriosus are wanting in Didelphys as in all other adult marsupials * as 
hitherto described by Owen, 66, Cunningham, 82, Rose, go, Parsons, 96, 
and Parsons and Windle, 98. Rose has explained the absence of the 
fossa and annulus ovalis in the heart of the adult marsupial on the 
ground that in the embryo the two auricles communicate with each 
other, as in birds and monotremes, by means of a number of small 
openings which are secondarily formed, and which close up early in 
correlation with the abbreviated intrauterine life of these animals. 
2. The Right Auriculoventricular Valve-—The right auriculoventricu- 
lar valve of Didelphys consists of one medial or septal and two lateral 
membranous cusps which are continuous at their bases round the 
auriculoventricular orifice. The left lateral cusp is the largest, the 
septal next, while the right lateral is quite small and is only with diffi- 
3 With the possible exception of Perameles in which an allantoic placenta is present. 
