xm PHYLUM CHORDATA 305 



end of the trunk towards the ventral aspect ; each, like that of the 

 Frog, consists of three divisions thigh or femur, shank or crus, 

 and foot or pes. The pes, like the manus, terminates in five clawed 

 digits, of which the first or hallux is the smallest. The head is 

 somewhat pyramidal, slightly depressed : the openings of the 

 external nares are situated above the anterior extremity. The 

 mouth is a wide slit-like aperture running round the anterior 

 border of the head. At the sides are the eyes, each provided with 

 upper and lower opaque, movable eyelids and with a transparent 

 third eyelid or nictitating membrane, which, when withdrawn, lies 

 in the anterior angle of the orbit. Behind the eye is a circular 

 brown patch of skin the tympanic membrane corresponding 

 closely to that of the Frog, but somewhat sunk below the general 

 level of the skin. The trunk is elongated, strongly convex dorsally, 

 flatter at the sides and ventrally. At the root of the tail on the 

 ventral* surface is a slit-like transverse aperture the anus or 

 cloacal aperture. The tail is cylindrical, thick in front, gradually 

 tapering to a narrow posterior extremity ; it is nearly twice as 

 long as the head and trunk together. 



There is an exoskeleton of horny scales covering all parts. 

 These are formed from folds of the dermis, each covered with a thick 

 horny epidermal layer. In size they differ in different positions. 

 On the dorsal surface of the trunk they are small, hexagonal, and 

 indistinctly keeled. On the ventral surface they are larger and 

 are arranged in eight longitudinal rows. Immediately in front of 

 the cloacal aperture is a large pre-anal plate. A collar-like ridge 

 of larger scales surrounds the throat. On the tail the scales are 

 elongated, keeled, and arranged in regular transverse (annular) rows, 

 giving the tail a ringed appearance. On the surface of the limbs 

 the scales of the pre-axial (radial or tibial) side are larger than 

 those of the post-axial (ulnar or fibular). The scales on the upper 

 surface of the head (head-shields) are large, and have a regular and 

 characteristic arrangement. 



Endoskeleton. The vertebral column is of great length and 

 made up of a large number of vertebrae. It is distinctly marked 

 out into regions, a cervical of eight vertebrae, a thoraco-lumbar of 

 twenty-two, a sacral of two, and a caudal of a considerable but 

 indefinite number. A vertebra from the anterior thoracic region 

 (Fig. 972, A, B) presents the following leading features. The 

 centrum (cent.) is elongated and strongly proccelous, i.e. the anterior 

 surface is concave, the posterior convex ; the neural arch beafs a 

 short neural spine (sp.). There are pre- and post-zygapophyses 

 (pr. zy, pt. zy), the former with their articular surfaces directed 

 upwards, the latter downwards. On each side at the junction of 

 centrum and neural arch is a facet the capitular facet for the 

 articulation of a rib. The cervical vertebra in general are similar 

 in essential respects to those of the trunk, but are somewhat shorter. 



