180 Dr. J. E. Gray on American Emydidfe. 



The genera Deirochelys and Graptemys have a single eye spot 

 under each dorsal shield, which is surrounded with narrow poly- 

 gonal rings, sending out anastomosing cross lines to the margin. 

 Deirochelys has a central continued narrow vertebral streak, not 

 found in Gi-aptemys, which is peculiar in having a nodulose ver- 

 tebral keel invested with oblong rings. 



The genus Chrysemys is at once known by the pale margin to 

 the dorsal shields, and the continued vertebral streak. 



Even the coloured lines on the fore legs seem to be charac- 

 teristic of genera. In Trachemys, for example, the upper streak 

 is continued on to the second toe ; and in Pseudemys it is bent 

 and continued on to the third or middle toe. 



The Nicotees, or West-Indian Emydoids, have a nearly uniform- 

 coloured back of the shell, with dark spots on the margin of the 

 shields. In the Catalogue (p. 31) I pointed out the difference 

 in the form of the head and skull of the two species, E. decussata 

 and E. rugosa. Professor Agassiz refers the first to the genus 

 Ptychemys, and the latter to Trachemys. They appear to be 

 rather aberrant species of these genera, without colour, at least 

 in the adult state. I have never seen young specimens of either. 

 Do they in that state show the pattern which is typical of these 

 genera ? for it is the younger animals that have the colours most 

 distinctly marked, and on which the disposition is best studied. 

 It is therefore more remarkable that Professor Agassiz should 

 make the statement that is here quoted, as he has studied these 

 animals in their young state, and figured the newly hatched 

 specimens of most of the North-American species ; but the pat- 

 tern, in some of the figures, is not so distinct as it might be, or 

 as it is in the specimens, even when they have been preserved 

 in spirits. 



In the Catalogue will be found several additional observations 

 showing the coloration of the species of these genera. 



The following are the genera and synonyma of the North- 

 American Emydoids : — 



Deirochelys, Agassiz, Contrib. i. 414 (1857). 

 D. reticularia, Agassiz, 414, t. 1, f. 44; 16, t. 2, f. 1-3 (young). 

 Emys reticularia, Gray, Cat. 27. 



Graptemys, Agassiz, Contrib. i, 436 {\d>o7).=-Emys, Sect. **§§ 

 Gray, Cat, Shield Kept, 29 (1855), 



1. G. geographica, Agassiz, Contrib. 436, t, 2. f. 7, 9. E. geo- 

 graphica, Gray, Cat. I. c. 29. 



2. G. pseudogeographica. Emys pseudogeographica, Gray, /, c. 

 29; Holbrook, t, 15. G, Lesueurii, Agassiz, Cont. p, 436, 

 t. 2. f. 10, 12. 



