178 



AMPHIBIA. 



vertebrae.* The number of vertebrae is usually considerable, in 

 accordance with the elongated form of body ; but in the Batrachia 

 the vertebral column consists of only ten vertebrae with very long 

 transverse processes, which usually at the same time represent the 

 ribs ; while, with the exception of tho first vertebra which is modified 

 to form the atlas, almost all the vertebrae of the trunk possess small 

 cartilaginous rudiments of ribs. The sacral region is formed by a 

 single vertebra (fig. 621). 



Skull. The primordial cartilaginous cranium persists, but usually 

 loses its roof and floor, and is partly replaced by bony pieces, some of 

 which are ossifications of the cartilaginous capsule (exoccipitals, 

 auditory capsules, sphen-ethmoid, quadrate), while others are 



Jmx 



V 



n 



FIG. 622. Skull of Sana esculenta (after Ecker). a, from the dorsal, b, from the ventral 

 side ; [Membrane bones of one side removed in each case]. Ocl, exoccipital ; Pe, petrosal 

 (prootic) ; JSt, girdle-bone or sphen-ethmoid ; Ty, tympanic ; Fp, fronto-parietal ; /, 

 quadrato-jugal (jugal); MX, maxillary; Jmx, praemaxUlary ; N, nasal; Ps, para- 

 sphenoid ; Pt, pterygoid ; Pi, palatine ; F, vomer. 



investing bones (parietals, frontals, nasals, vomer, parasphenoid} 

 (fig. 622). As in Lepidosiren the basi- and supra -occipital remain 

 as small cartilaginous tracts. There is also a parasphenoid on the 

 base of the skull (fig. 622, Ps). The large exoccipitals (Ocl) (fused 

 with the opisthotic) articulate by means of two condyles with the first 

 vertebra, as in the Mammalia. The projecting auditory region is 

 pierced by the fenestra ovalis, and the bone in its anterior part 

 corresponds to the prootic (Pe). The lateral walls of the skull 

 remain cartilaginous, but in the ethmoid region there is a ring- 

 shaped bone the girdle bone, or sphen-ethmoid. 



* [For a fuller account of the development of the Amphibian vertebral 

 column, vide Balfour, " Comparative Embryology," vol. ii., p. 456 ct ~ 



