374 Venous System of Didelphys Marsupialis 
Fami ly—DasyURID&. No. of Cusps. No. of musculi papillares. 
Thylacinus cynocephalus. 5 Two groups. 
(CUNNINGHAM, 82.) 
Thylacinus cynocephalus. 3 One large, one medium- 
(ROSE, go.) sized and several small 
Dasyurus viverrinus. 2 3 
(CUNNINGHAM, 82.) 
Phascogale calura. Not given. 2 
(CUNNINGHAM, 82) 
Family—DIDELPHYID. 
Didelphys marsupialis. 3 3 
(McCLuRE. ) 
From the above table, which is supposed to represent the normal con- 
ditions, it is seen that the number of cusps that may enter into the 
formation of the right auriculoventricular valve of marsupials ranges 
between two and five, and that in one instance (Z’hylacinus) there ap- 
pears to be a marked difference of opinion as to the number of cusps 
present. The question naturally arises: Do those cases in which less or 
more than three cusps have been observed represent the normal con- 
ditions, or do they indicate that the observers have interpreted differ- 
ently as to what really constitutes a cusp? It is well known that con- 
siderable variation exists among the higher mammals so far as their 
valve structure is concerned. In some mammals there is scarcely any 
division into a right and left lateral cusp, so that one would not be far 
from the mark in describing their valves as consisting of one lateral 
and one septal cusp. In other mammals the three cusps may be quite 
distinct and in some instances an extra or supernumerary cusp may be 
present, being formed, probably, as the result of an extra notching of 
the free border of one of the lateral cusps. In view of the above state- 
ments it appears to the writer that the membranous valves of marsupials 
must be classed with those of the higher mammals, a classification 
already adopted by authors of recent text-books (Beddard, o2, and 
Wiedersheim, 02), although they have given no reasons for so doing. 
Before closing this topic it may be stated that the semi-lunar valves 
of the aorta and pulmonary artery, as well as the mitral or bi-cuspid 
valves of Didelphys and other marsupials, thus far described, agree in 
their structure with those of the higher mammals. . 
3. The Pulmonary Veins.—There is little to be said concerning the 
pulmonary veins of Didelphys except that they usually unite to form 
a V. pulmonalis communis before opening into the left auricle. In a 
few cases in which a V. pulmonalis communis was not present the veins 
always opened into the auricle close together. 
