154 ZOOTOMV. 



by five clawed digits ; of these the first is the shortest, 

 the second and fifth are of nearly equal length ; the fifth, 

 however, being the slenderer, and appearing shorter from 

 the fact that it arises nearer the proximal extremity of 

 the hand ; the third is considerably longer than the second, 

 and the fourth slightly longer than the third (cf. 107 



-us)- 



119. The large mouth, the gape beginning at the end of 

 the snout and extending backwards nearly to the auditory 

 apertures ( 122). 



120. The nostrils, or anterior nares, small, rounded 

 apertures, situated one on each side of the snout, a short 

 distance from its anterior end. 



121. The eyes, situated one on each side of the head, 

 about midway between the corresponding nostril and 

 auditory aperture ( 122) : each is guarded by a short, thick 

 upper eyelid, and a deep, thin lower eyelid, which, 

 when the eye is closed, covers the greater part of it : within 

 the eyelids is the thin, transparent nictitating membrane, 

 which springs from the anterior corner of the eye and shuts 

 backwards over the eye-ball. 



122. The auditory aperture, situated a little posterior 

 to the extremity of the gape, and nearly as large as the 

 exposed portion of the eye : stretched over it, a little belo\v 

 the general level of the head, is the thin tympanic 

 membrane. 



123. The vent, or cloacal aperture (Fig. 39, <-/), a 

 large transverse cleft at the root of the tail. 



124. The exoskeleton, consisting of a closely-fitting 

 armour of epidermic scales : those on the dorsal surface 

 of the trunk are small, hexagonal in outline, and indistinctly 

 keeled ; on the ventral surface they are large and are arranged 

 in eight longitudinal series ; forming the anterior edge of the 



