OSTEOLOGY 153 



Metacarpus. The metacarpals number five. The first, 

 belonging to the pollex, is very short but extends, with the 

 second metacarpal, farther proximad than any of the others. 

 The head of the latter has a medial fossa for articulation 

 with the multangulum minus. The proximal terminations 

 of the third and fourth are situated consecutively farther dis- 

 tad, as is the medial portion of the fifth, but of the last 

 there is a lateral extension proximad for articulation with the 

 hamatum. The longest of the metacarpals is the third. 

 Taking this as representing 100, we get the following values 

 from the others. In Homodontomys the first is 41; the 

 second 82; the fourth 92; and the fifth 75. In Neotoma the 

 first is 27 ; the second 83 ; the fourth 92 ; and the fifth 73. In 

 Teonoma the first is 43 ; the second 84; the fourth 93 ; and the 

 fifth 75. There is thus very slight variation in the relative 

 length of the metacarpals, save that the pollex is the most 

 rudimentary in the least scansorial subgenus. 



The flexor carpi radialis is inserted upon the pahnar aspect 

 of the base of metacarpal two. Upon the dorsal surface of 

 the manus, the extensor metacarpi pollicis has insertion upon 

 metacarpus one, the extensores carpi radialis longus and 

 brevis respectively on the second and third, and the ex- 

 tensor carpi ulnaris upon the fifth. 



Digits. There are two phalanges upon the pollex or first 

 digit, although the terminal one is but a minute nubbin, 

 without a nail. There are three phalanges to each of the 

 remaining four digits, although the terminal ones, from 

 which extend the claws, are so small and surrounded in the 

 usual cleaned specimen by dry cartilaginous tissue that it is 

 difficult to define them. 



Due to the extreme tenuousness of many of the digital 

 tendons and the great difficulty experienced in separating 

 them from their connective tissue, it is impossible to be 

 certain regarding the exact point of insertion upon the 

 phalanges of some of them. This has been illustrated as 

 exactly as possible, however, in the accompanying figures. 



