EXTERNAL CHARACTERS 13 



the angle is termed the "canthus rostralis," which 

 may be intensified by the loreal region being concave. 



The deep pits which are sometimes present on 

 the lips or between the nostril and the eye (loreal 

 pit) will be alluded to further on under Sensory 

 Organs. 



The nostrils are either lateral, or, in the aquatic 

 forms, directed upwards, sometimes entirely on the 

 upper surface of the snout. 



Most snakes have a longitudinal groove on the chin 

 (mental groove) to allow for the distension caused by 

 the lateral movements of the rami of the lower jaw. 



In the Typhlopidse, the head passes gradually into 

 the vermiform body, and the small mouth is situated 

 on the under surface of the projecting snout ; the 

 head so resembles the extremely short tail, and the 

 mouth is so similar in shape and position to the vent, 

 which is close to the posterior extremity of the snake, 

 that such creatures are often believed by non-critical 

 observers to have a head at each end. The eyes are 

 very small, and covered over by the semi-transparent 

 head-shields, or they may be completely concealed. 

 There is no mental groove. It is much the same 

 with the Glauconiidae, which have, however, a some- 

 what less abbreviated tail. In both, the nostrils 

 often open on the lower side of the snout, which may 

 be excavated so as to appear hooked in profile, or 

 may be provided with a sharp cutting horizontal 

 edge. 



