IlSrTRODUCTORY LETTER. 
9 
was necessary to separate the Hydrophis Colubrina from 
its congenersj and to arrange the Acrochordus among the 
Sea Snakes, &c. 
You will 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 
Bhodostoma 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 
