34 THE ANATOMY OF VEETEBEATED ANIMALS. 



wliile tlie corresponding sui-face of the leg looks backwards 

 and upwards, and tlie ungual phalanges are tiu'ned back- 

 wards. 



The chief modifications of the manus and pes arise 

 from the excess, or defect, in the development of particular 

 digits, and from the manner in which the digits are con- 

 nected with one another, and with the carpus or tarsus. In 

 the Ichthyosauria and Plesiosauria, the Tm-tles, the Cetacea 

 and Sirenia, and, in a less degree, in the Seals, the digits are 

 bound together and cased in a common sheath of integu- 

 ment, so as to form, paddles, in which the several digits have 

 little or no motion on one another. 



The fourth digit of the manus in the, Pterosauria, and 

 the four ulnar digits in the Bats, are vastly elongated, to 

 support the web which enables these animals to fly. In 

 existing birds the two ulnar, or post-axial, digits are aborted, 

 the metacarpals of the second and third are ankylosed to- 

 gether, and the digits themselves are inclosed in a common 

 integumentary sheath ; the thii'd invariably, and the second 

 usually, is devoid of a claw. The metacarpal of the pollex 

 is ankylosed with the others, but the rest of that digit is 

 free, and frequently provided with a claw. 



Among teiTCstrial mammals, the most striking changes of 

 the manus and pes arise from the gradtial reduction in the 

 number of the perfect digits from the normal number of five 

 to four (Bus), tkree [Rhinoceros), two (most Buminantia), or 

 one (Equidce). 



The Pectoral and Pelvic Arches. — The proximal skeletal 

 elements of each pair of limbs (humeri or femora) are 

 supported by a primitively cartilaginous, pectoral, or pelvic 

 girdle, which lies external to the costal elements of the 

 vertebral skeleton. This girdle may consist of a simple 

 cartilaginous arc (as in the Sharks and Rays), or it may be 

 complicated by subdivisions and additions. 



The pectoral arch may be connected with the skull, or 

 with the vertebra] column, by muscles, ligaments, or dermal 

 ossifications, though, primitively, it is perfectly free from 



