PEOPESSOR OWEN ON THE SPECIES OF PHASCOLOMTS. 493 



A pair of strong sesamoids (fig. 6, s, s) increase the force of the flexor tendons as they 

 pass over the proximal joints of the digits. A flexor process (f, figs. 6 & 7) adds to the 

 leverage of insertion beneath the base of the ungual phalanges ( 3 ). 



Pelvis. — The os innominatum in Phascolomys (PL LXXIII. figs. 1-6) is about one 

 fourth the length of the entire skeleton : the leverage it aff'ords to the limb-muscles by 

 the extension of the ilium (02, a) in advance of the acetabulum {t), and by the extension 

 of the ischium (es, h, li!) behind that centre of motion, is considerable and adds greatly 

 to the power of those muscles. 



The ilium, long, slender, subcompressed, and almost subprismatic, is twisted at right 

 angles to the ischium, the plane of its expanded fore end (a, I) looking vertically, while 

 that of the ischium (//, h!) looks laterally, in the standing pose of the skeleton. 



The dorsum ilii (PL LXXIII. figs. 1 & 2, 62') is directed upward with a slight obliquity 

 backward ; it is divided into an upper facet (62, d 62) marked off from the hinder facet 

 (62'), by a longitudinal rising parallel with the upper or sacral border {h,u). The part 

 corresponding to the 'internal iliac fossa' of Anthropotomy is a short triangular tract 

 (fig. 4, 62») which is continued to the apex (a) of the outer production of the ' labrum.' 

 About 2 inches of this expanded part of the ilium (figs. 1 & 2, a, b) is in advance of the 

 sacral articulation {p, u, fig. 1, and ]), fig. 3). The hind end of this part answers to the 

 post-inferior spine (figs. 1 & 2, u) of the human ilium, and to the beginning of the ' great 

 sacro-sciatic notch' (m, I, figs. 1 & 2). The dividing angle of the dorsal surface is con- 

 tinued to the tuberosity (d) answering to the ' ridge of the reflected tendon of the 

 rectus muscle'? in Anthropotomy, but which here gives attachment to the sole origin of 

 the ' rectus femoris muscle''. 



The anterior border or ' crest' of the ilium (a, h, c) is obtuse, thick, and rough, 

 broadest at the produced angle (a) answering to the ' antero-superior spine' of Anthro- 

 potomy. The length of the ' crest ' is less than half of that the entire ilium. 



The mesial or sacral surface of the ilium is smooth and uniform, gently concave 

 across, in a less degree convex lengthwise. The joint for the sacrum is limited to its 

 hinder part, is reniform (PL LXXIII. fig. 5), and is divided into two fiat, oval or oblong 

 facets {p 1, ]} 2) for the pleurapophyses of the two anterior sacral vertebrae. 



Below this surface the ilium contracts to a strong three-sided prism (z) with the angles 

 rounded ofi', and then expands to form the anterior half of the acetabulum {t), where it 

 coalesces with the pubis (64) and ischium (63). At this junction with the pubis is deve- 

 loped the ilio-pubic process (fig. 3, e). 



The pubis is compressed, especially at its symphysial part (I) ; the oblong ridge (k) for 

 the articulation of the marsupial bone (fig. 7) terminates near the angle at which the 

 symphysis begins. 



The ischium, contributing the lower half of the acetabulum (i), is strong and 



' This tuberosity is not the homologue of the ' anterior inferior spine ' of the ilium (see ' Annals and Magazine 

 of Natural History,' 1870, vol. v. p. 168. 



TOL. VIII. — PAKT VIII. April, \?>li. 3z 



