EPIDERMAL APPENDAGES. 319 



angular form of the vlperine head Is here noteworthy. In 

 some of the Tropical American viperine species (the 

 CrotalidcB) the angular head is so marked as to be separated 

 into a genus — the Trigonocephali, three-cornered heads. 

 One head is sagittate or arrow-shaped to such an extent 

 that the serpent is known as the Fer de lance, the dreaded 

 Trigonocephahis lanceolaUis of the Antilles. There are 

 Trigonocephali among the Indian Thanatophidia also. 



One other very remarkable exception must not be 

 omitted — namely, that in pythons may be seen an angular 

 head, which makes the neck thin and conspicuous, only 

 in a less degree ; and also the absence of large head shields. 

 In addition to this, many of the pythons have particularly 

 short and very pointed tails — three singular viperine features 

 in non-venomous snakes, which can only be inherited from 

 a common ancestry. 



Another caprice is seen in the carinated or keel-shaped 

 body scales, which are found in venomous and non-venomous, 

 land and water, ground and tree snakes indifferently ; 

 though I think one may be safe in affirming that none 

 of the true vipers have unkeeled and polished scales. 

 Nicholson has observed that in several allied species, some 

 have and some have not the keel, and that those without 

 do as well as those with. 'The history of the keel is not 

 known,' says this author. In appearance it reminds one 

 of the mid-rib of a leaf or of a feather, and may probably 

 be an inherited feature in common with birds whose 

 reptilian ancestry in process of ages had fluttered their 

 scales into feathers. In fact, in many snakes where no keel 

 is found, there is some slight indication of a centre line, even 



