174 



ELEMENTAR Y ANA TO MY. 



[LESS. 



The first may disappear or be rudimentary, as happens in 

 very many forms ; or the second digit alone may be very 

 reduced, as in the Potto. 



The first may be short and the four others exceedingly 

 elongated and slender, as in the Bats ; 

 or the first three may be short and the 

 fourth enormous, as in the Pterodactyles. 

 The whole five metacarpals may be 

 extremely short and stunted, as in the 

 Mole, and still more so in the Land Tor- 

 toises and in the Ichthyosaurus. 



The number five is never increased 

 (except by monstrosity) in any known 

 Vertebrate. 



21. The number of DIGITS which may 

 be developed has been indicated in 

 treating of the metacarpus. Thus there 

 may be but a single digit, as in the 

 Horse; two, as in Ruminants, and in 

 Chaeropus. There may be three, as in 

 the Rhinoceros, Proteus, and Seps 

 (second, third, and fourth, or first, 

 second, and third) ; or, as is very often 

 the case (e.g. Pig, Dog, &c.), there may 

 be four. 



The digits are never certainly more 

 than five (except by monstrosity), al- 

 though in the Ichthyosaurus extra mar- 

 ginal bones give the appearance of more. 

 The filamentary bones which terminate 

 or fringe the fins of Fishes are not digits 

 at all, but are dermal ossifications called 

 " fin rays," which will be spoken of with 

 the rest of the external skeleton. 



When a digit is wanting, it is generally 

 the pollex, as in Spider Monkeys, but it 

 may be the fifth, as in Pterodactyles, or 

 the fourth and fifth, as in Birds. 



The thumb (pollex) may be more or 

 less opposable, as in most Monkeys and 

 Lemurs ; or two digits may be opposed to three, as in the 

 Chameleon. 



The second digit may be greatly reduced, as in the Potto ; 

 or the third may be disproportionately slender, as in the Aye- 



FIG. 148. HAND OF 

 BAT (Pteropns). 



m * in 4 , metacarpals of 

 the four fingers ; p, 

 pollex, with a very 

 short metacarpal, sc, 

 scaphoides. 



