INTRODUCTORY LETTER. 9 



was necessary to separate the Hydropliis Colubrina froiii 

 its congeners, and to arrange the Acrochordus among the 

 Sea Snakes, &c. 



You ^vill perceive, in the sequel of my work, that it is 

 absolutely impossible to class Ophidians from such divided 

 and isolated characters. This subject, however, is of too 

 much importance to be thus passed over. I must satisfy 

 myself, by quoting some more examples calculated to defend 

 my ideas against the objections of my adversaries, although 

 I fear that I have already exhausted your patience. All 

 the world allows, that the genus Dryiophis is one of the 

 most natural of the whole order : it may be indicated by 

 distinctive features, taken either from its muzzle drawn 

 out into a tube, or from the superior length of its middle 

 and posterior maxillar teeth ; either from the transversely 

 elongated pupil of its eye, or from its green colour, or, 

 lastly, from its smooth scales. But none of these features 

 are, at the same time, applicable to all the species. The 

 fixed essential character of the Najas is an extensible neck ; 

 but, in the different species that compose this genus, the 

 faculty of dilating the neck is possessed in all degrees, so 

 that those which recede most from the type, scarcely ex- 

 hibit traces of this character. Almost all the venomous 

 serpents, properly so called, have carinated scales ; but no 

 person would remove from that family the Trigonocephalus 

 Rhodostoma and Tr. nigro-marginatus, because their scales 

 are smooth ; no one would reject from the family of Co- 

 lubriform venomous snakes, of which the scales are gene- 

 rally smooth, the Naja Haemachates and N. rhombeata, in 

 which the reverse is the case. We observe in the first 

 family a small number of species, the head of which, 

 covered with plates, approximates them to the second fa- 

 mily, although in all the other characters they resemble 

 the first. Could one mistake the affinity that exists be- 

 tween the Boa and Acrochordus, although the latter has a 

 compressed tail, and wants the anal hooks ? What confu- 

 sion has arisen from the innumerable individual differences 

 in the disposition of the plates of the head in the Boas and 

 the Pythons ! Neither the position of the nostrils, nor 

 the configuration of the frontal plates, nor the presence 



