OF DIDELPHYS VIRGINIANA. 83 



extension upward in front of the acetabulum. The ilium, after expanding to form its part 

 of the acetabulum, narrows for a little, without decided ridges or borders, and then suddenly 

 becomes sharply prismatic ; which shape it preserves to its tip. The outer and posterior 

 faces of the prism are perfectly smooth — the former^decidedly concave : the inner (which is 

 the broadest) is roughened in the usual manner for the sacro-iliac synchondrosis. The 

 apex of the ilium is slightly clubbed, smooth and convex. The shaft is almost perfectly 

 straight. With the exception of the sacral portion, the whole internal aspect of the in- 

 nominate l)one is smooth, and devoid of decided prominence. The two sides of the pelvis 

 meet at the pubis, at an angle of less than 90°. 



It is only necessary to allude, in this connection, to the great size of the pelvic outlet 

 that obtains in this, as in other marsupials, compared wdth the dimensions of the foetus at 

 term. The disproportion is, however, less than that of some Marsiqnalia salientia ; 

 though greater than that wdiich obtains in the Petaurists, wdiere, according to Owen, the 

 diameter of aperture is six times that of the foetal head. Aside from other pelvic charac- 

 ters common to the order, (such as length, narrowness and flatness, extreme modification 

 of ischio-sacral notch, extensive pubic symphysis, etc.) the length and straightness of the 

 ilia, flatness of the ischia and pubes, and a general deficiency of well-marked angles or 

 processes, such as an ilio-pubic or a backward, curved extension of the tuber ischii, are 

 among the most notable points. The mammalian pelvis here finds very simple expression. 

 The shape of the pelvis appears to relate mostly to the prehensile function of the tail, and 

 the arm-like movements of the posterior extremities — to which latter it is peculiarly 

 adapted. 



Ossa marsupialia. — (Fig. 24.) The characteristic adjuncts of the marsupial pelvis here 

 appear of moderate proportions, being neither very large nor very small, and of simple 

 form. They are an inch and a quarter or half long, and ordinarily stand a little outward 

 and forward, diverging from each other, their bases almost in mutual contact. In the 

 accompanying figure, this obliquity would cause the bone to be much foreshortened were it 

 drawn in situ ; it is accordingly dislocated and laid flat in the plane of the rest of the 

 pelvic wall. The shaft is flattened, smooth, Avithout decided processes, very slightly 

 curved (the convexity outw^ard), slightly knobbed at the apex, spreading below into a later- 

 ally expanded foot. The margin of the base is oblique to the axis of the shaft, pro- 

 longed to meet its fellow at the symphysis pubis, and giving off externally a rounded 

 knob or claw which lies upon, though not articulating wnth, the pubic ramus. The articu- 

 lating facet is flat and narrowly oval, loosely fitting the tuberosity on the pubis already 

 noticed. The joint is a simple hinge, permitting only flexion and extension of the bone 

 upon the pubis ; the amount of motion enjoyed is considerable. The mutual rehition of 

 the planes of the two articulating surfaces is such, that wdien the bones are drawai toward 

 the abdomen their apices are approximated ; and conversely. 



The marsupial bones do not present any notable diflerences in the two sexes. In the 

 male, they are thought to be subservient to the action of the cremaster, but as this muscle 

 passes across them but little above their bases, the greatest outward inclination that they are 

 capable of would but slightly affect the course, and consequently the traction, of this mus- 

 cle. In the female, they must, I think, further the advantageous action upon the pouch of 

 the muscles of the abdomen, even if they do not actually increase the power of the latter. 

 Their relation to the proper muscle of the pouch may be more intimate and important. 



