xvi.] GENERAL CHARACTERS. 257 



may be in a very rudimentary condition, or altogether 

 suppressed. If one is absent, it is most commonly the 

 first. 



Excepting the Cetacea, no Mammals have more than three 

 halanges to each digit, but they may occasionally have 

 wer by suppression or ankylosis. 



The first or radial digit (also called pollex) is an exception 



the usual rule, one of its parts being constantly absent, for 



hile each of the other digits has commonly a metacarpal 



d three phalanges, it has only three bones altogether. 



hether the missing one is the metacarpal or one of the 



halanges, is a subject which has occasioned much dis- 



ssion, but has not yet been satisfactorily decided. In 



cordance with the most usual custom, the proximal bone 



this digit will here be treated of as a metacarpal. 1 



The terminal phalanges of the digits are often specially 



odified to support the nail, claw or hoof, and are called 



ungual phalanges." 



Very frequently a pair of small sesamoid bones are 

 veloped in connection with the tendons passing over the 

 lmar surface of the articular heads of the metacarpals and 

 phalanges, and occasionally (as in the Armadillos) a larger 

 bone of similar nature is met with in the middle of the same 

 surface of the carpus and metacarpus. More rarely similar 

 bones occur on the dorsal surface of the phalangeal articu- 

 lations. 2 



Each of the carpal bones ossifies from a single nucleus. 

 The metacarpals and phalanges have each a main nucleus 



Ir the greater part of the bone, and usually an epiphysis at 

 ra 



1 See Allen Thomson " On the Ossification of the First Metacarpal 

 >ne." (J own. Anat. and Physiology, Vol. III. p. 131.) 



2 These are almost always lost in prepared skeletons, but they occur 

 istantly in the common dog. 



