OF DIDELPHYS VIRGINIANA. 81 



fino-er metacarpals are notable for the enlargement of the heads, as compared with the 

 bases • the slenderest point is just beyond the base, whence the bone gradually enlarges to 

 the clubbed extremity. The thumb metacarpal differs in being larger at the base than at 

 the head and in having the constriction nearer the distal extremity. The second — fourth 

 metacarpals lie as nearly parallel to each other as the difference in the size of their bases 

 and heads will permit ; these are both in contact, the f()rmer by articulation, the latter by 

 simple apposition. The first, on the contrary, stands away from the others at an open 

 ano-le ; and this divergence is indicated as clearly by the articulating f^xcet upon the side 

 of the base of the bone, as by the outward shape of the hand. The facet in question is 

 small and slightly cupped, and situated in a plane only a little oblique instead of perpen- 

 dicular to, and directly continuous with, that for the carpal articulation — that is, upon the 

 top of the head of the bone, rather than upon tlie side. It fits a little protuberance upon 

 the corner of the base of the second metacarpal, and has motion in more than one direc- 

 tion, much freer than that enjoyed by the others. The head of the second metacarpal 

 is divided by a vertical groove into two parts, with rather sharp and somewhat produced 

 edges, one of which bears the facet for the first metacarpal just noticed ; the other projects 

 over the base of the third, so that the latter has carpal articulation only by half its head, 

 the other half being received into a depression in the side of the head of the second. The 

 head of the fourth, slightly convex and scarcely grooved, is flush with the corresponding 

 half of the head of the third ; the two are mutually apposed by a small plane facet. The 

 head of the fifth is again flush with that of the fourth ; it is made somewhat triangular by 

 a small pointed process on its distal side, separated superiorly by a decided groove from 

 the rather convex facet by which it articulates with the fourth. All these inequalities of 

 the heads of the bones as one row correspond, it need hardly be added, to irregularities in 

 the surfiice of the contiguous carpal row. The metacarpals are depressed along their shafts, 

 being wider than thick ; they are straight, or nearly so, but downward production of both 

 bases and heads render their palmar profile concave ; which arching, together with the con- 

 cavity of the hand from side to side, produces a decided palmar hollowing. The heads of 

 all the bones are very convex, wider than deep, and present a slight median vertical ridge 

 bounded on each side by a corresponding slight groove; these inequalities are most 

 marked on the first. 



Phalanges. — There is scarcely any difference in the lengths of the second — fourth ; the 

 claw of the fifth reaches the base of that of the fourtli ; the claw of the thumb only 

 reaches to the base of the middle phalanx of the index finger. The fingers have each 

 three phalanges, whereof the first makes up most of their length, equalling, or exceeding, 

 the otlier two and the claw together. The thumb has two ; the proximate one is as long 

 as the distal one and its claw. The ungual phalanx of each finger and the thumb is sim- 

 ply a little nodule supporting the claw— the thumb, unlike the great toe, being unguiculate 

 like the other digits. The other phalanges have the usual configuration, and present no 

 noteworthy peculiarities. 



Sesamoids. — There are ten sesamoids in the hand — two small oval nodules placed length- 

 wise, side by side, beneath the metacarpo-phalangeal articulation of each of the five 



digits. 



MKMOIRS BOST. 800. NAT. IHST. VOI,. II. 21 



