xu INTRODUCTORY LETTER. 



nor absence of a gi'ooved tooth, are constant characters in 

 the genus Ilonialopsis. The late M. Boie has assigned to 

 the Tropidonotus three posterior ocidar phites ; 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 tjlj}G, 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 affinities. 



I have also added to my work several charts, and a 

 table intended to indicate the distribution of the species 

 of Ophidians over the sm-face 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, 

 ^n 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 



