INTEGUMENT 17 



been by a modification of the character of the seg- 

 ments about the anal region, a conversion of trunk 

 vertebrae into caudals, or vice versa. In dealing with 

 certain species — of Vipers, for instance — it is impor- 

 tant, for systematic purposes, to keep the counts of 

 shields distinct for the two sexes. 



The shield which covers the vent, the anal shield, 

 is either single or divided into two. 



Some snakes have the head covered with scales or 

 small tubercles similar to those on the body, but in 

 the great majority the lepidosis is in the form of 

 large symmetrical juxtaposed shields, the shape, pro- 

 portions, and number of which furnish some of the 

 most important characters for the distinction of 

 genera and species. These head-shields belong to 

 two primarily different types, from each of which all 

 further modifications may be regarded as derived by 

 alteration in shape or by disintegration. The first 

 type is that shown by the Typhlopidae and Glau- 

 coniidas, which is explained by the figure on the 

 next page. 



The rostral, which is usually the largest of the head- 

 shields, extends to the upper surface of the head, of 

 which it may occupy the greater part. In the 

 Glauconiidae, the ocular usually borders the mouth. 



As may be seen by a comparison of the first figure 



with the second, the arrangement of the head-shields 



is essentially different from that which prevails in the 



Colubrids and the majority of other snakes. 

 2 



