492 PEOFESSOB OWEN OX THE SPECIES OF PHASCOLOMTS. 



to the reception of which the pisiform (])) also contributes ; the cuneiform supports the 

 rest of the pisiform, and articulates with the lunar part of the scapholunar and with the 

 unciform. 



The pisifonn (ib. fig. 5, p) is a strong, thick, subelongate bone, expanded at both ends ; 

 the proximal one ofiers two articular surfaces for the ulna and cuneifonn, the distal end 

 is simply convex and smooth ; it might be mistaken for a proximal phalanx, to which 

 it has a general resemblance ; but the form of the two facets of the proximal surface 

 distinguishes the pisiform. 



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 ii.). 



The trapezoides (z) supports the metacarpal of the index (?n ii:) 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 (in) pushes its head between the lunar part of Ihe scapholunar (I) 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 

 oifers 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 u.). 



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 sui-face, and touches, there, the outer extension of the base of the mid 

 metacai-pal. 



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

 between the trapezium and pollex, on one side, and between the magnum (in) 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 iii.) 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 (ii.), 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 III., IV., V. ; they are depressed and subtruncate at the free end on each side the 

 lever of insertion (fig. 7, 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. 



