10 
IlSTTEODUCTOEY LETTEE. 
nor absence of a grooved tooth, are constant characters in 
the genus Homalopsis. The late M. Boie has assigned to 
the Tropidonotus three posterior ocular plates ; yet some 
species among the best characterized depart from the rest, 
by the absence of this distinctive mark, while it exists 
among the true Colubri, that present the characteristic 
indicated by M. Boie. A great number of genera in the 
division of innocuous serpents, comprising many species, 
have grooved teeth ; whilst the dentary system of the other 
species is often very uniform. 
The order in which I have arranged the species is not 
arbitrary. On comparing the species of a generic group, 
we may remark that the characters of the genus may be 
particularly decided in one of them ; which may be termed 
the typical species of that generic group ; but all the rest, 
though formed on the same type, present modifications 
more or less conspicuous ; some of them may depart from 
that type, to approximate to a neighbouring group, to which 
they serve as the transition. I have, in the descriptive 
part of my work, treated of species in the order I have 
just indicated. A Synoptic Table, which I have added to 
my work, will facilitate the review of the species ; it will in- 
dicate the mutual resemblances that connect them with each 
other ; in a word, it will point out their natural afiinities. 
I have also added to my work several charts, and a 
table intended to indicate the distribution of the species 
of Ophidians over the surface of the globe. The object 
which I proposed to myself in making them public, is 
solely to give a general sketch of the geographic distri- 
bution of Ophidians ; those who wish to be more minutely 
informed whether such or such a species exist in a parti- 
cular description, have only to consult the second part of 
my work, in which they will find the necessary information. 
I have also treated of this subject in a dissertation, entitled, 
An Essay on the Geographic Distribution of Serpents^ 
which is printed as a sequel to my work. It is unneces- 
sary, Sir, to say to you, that this work has cost me some 
precious moments. Almost entirely limited to my own 
researches, from the total deficiency of works furnishing 
an enumeration of the species of Ophidians of certain 
