INTRODUCTION. 645 
the following classifications of Dumeril and Bibron of Ophidia and 
Lacertilia, and Gtinther of Anoura, and Agassiz, as modified by Cope, of 
Testudinata, are perhaps the best attainable. 
That many of the marks used in describing species in this treatise are 
not constant, the writer and every student of Herpetology well knows. 
Thus the presence and absence of a loral or ante-orbital plate may both 
be seen in the same animal on opposite sides of the head; the number 
of rows of dorsal scales varies in different individuals of a species; 
the coloration and arrangements of the spots and stripes, the number of 
the upper and lower labials, in fact the cephalic plates are liable to 
become more or less fused and run together; the number and arrange- 
ment of the shields in the carapax, and even the shape of the head is 
more or less variable. In using such marks, for the purposes of 
description, the writer but acknowledges the imperfections of this 
branch of Zoology, and hopes that after this suggestion, no one will be 
misled by any of these variable characters at times used in the synopses, 
but that they may be found of service in the identification and study o 
the species. 
One of the most difficult things about the study of these animals is 
that their colors change so much when placed in alcohol. Thus yellow 
becomes white; green, blue; and red, brownish-black; while brown and 
metallic tints remain for some time unchanged. However, exposure to 
the sun for a season will often enable us to form an idea of the original 
shade. 
Sex may be told in most if not all Turtles by the males having convex 
plastra,and in the Anoura by the males, in the greater portion of the 
species, being supplied with vocal vesicles. 
