365 



The trapezium (t) presents a concavity to the scaphoid (si), a convexity to the meta- 

 carpal of the pollex (m I.), and a concavity for the contiguous border of the base of the 

 metacarpal of the index (m il). 



The trapezoides (z) supports the metacarpal of the index (m on the major part of 

 its distal surface, and applies the rest to the radial extension of the magnum (m) ; it 

 also articulates with the trapezium and the scapholunar. 



The magnum (m) pushes its head between the lunar part of the scapholunar (/) and 

 the unciform (u), so as almost to touch the cuneiform, but leaves a small part of both 

 those bones for mutual union, as in the Beaver. The expanded distal end of the magnum 

 offers a very slight concavity to the broad base of the third metacarpal (m in.) ; it also 

 contributes a facet to part of that of the index (m II.). 



The unciform (u) touches the ulnar margin of the scapholunar (si), presents a triangular 

 convexity to the cuneiform (cu), mainly supports the metacarpals of the fourth and fifth 

 digits at its distal surface, and touches, there, the outer extension of the base of the mid 

 metacarpal. 



The metacarpal of the index (m n.) has its base extended ' proximad,' to be wedged 

 between the trapezium and pollex, on one side, and between the magnum (m) and medius 

 metacarpal, on the other side, but is mainly applied to the trapezoides (z). 



The outer or ulnar part of the base of the mid metacarpal (m in.) is similarly extended 

 and interposed between the base of the fourth metacarpal and the magnum (m), so that 

 the three chief metacarpals overlap, interlock, or wedge each other firmly into their 

 places, adding, of course, to the strength and power of resistance of this main part of 

 the fossorial foot. The comparatively narrow base of the metacarpal of the ' minimus' 

 (m v.) is wedged between that of the fourth metacarpal and the unciform (u). 



The numerical character of the digital phalanges adheres to the mammalian formula. 

 The proximal ones of the three outer digits are shorter in proportion to their breadth 

 than those of the thumb (i.) and index (il), as are also the second phalanges (2). The 

 ungual ones ( 3 ) are larger than the two preceding phalanges ( 2, 1 ) in the three outer 

 digits in., iv., v. ; they are depressed and subtruncate at the free end on each side the 

 lever of insertion (fig. l,f) of the flexor perforans tendon; they are subdepressed and 

 subtruncate at the free end, near which the bone shows many small vascular perfora- 

 tions, indicative of the rich supply of material for the quickly worn digging claws. 



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 (/', figs. 6 & 7) adds to the 

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



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

 the length of the entire skeleton : the leverage it affords to the limb-muscles by the 

 extension of the ilium (62, a) in advance of the acetabulum (t), and by the extension of 

 the ischium (63, h, h') behind that centre of motion, is considerable and adds greatly to 

 the power of those muscles. 



35* 



