Dr. J. E. Gray on American Emydidse. 179 



grown ; for in tlie full-grown and more adult or aged specimens 

 the colour is apt to become suffused, and the distinctive character 

 of the pattern more or less obscured, or rendered more difficult 

 to analyze ; and I am satisfied that the best specific characters 

 of the species are to be derived from such a study. 



But not only does the pattern afi'ord good specific characters, 

 but, as far as I have been able to examine them, they seem to 

 give some of the best characters to separate the species into 

 natural groups, either genera or subgenera, as the student may 

 be inclined to regard them. Thus the best character for the 

 group of which E. ornata may be considered as the type is fur- 

 nished by the fact that there is one eye spot under each shield ; 

 and an excellent character to separate the species is the position 

 which this spot occupies on the shield in the young and the 

 older specimens, as marked in my Catalogue above cited (p. 24). 



As examples of the assistance which the distribution of the 

 colouring-matter, or the pattern, affords in the distinction of the 

 genera, I may observe : — The underside of the margin of most 

 coloured American Emydoids has a series of eye spots. In 

 Graptennjs the centre of this spot is on the hinder margin of 

 each of the marginal shields ; in all the other genera it is on the 

 suture between two neighbouring marginal shields, the spot 

 being on the middle of each of the marginal bones, and the 

 suture of the horny shields alternating with the suture of the 

 bones. The genera Chrysemys and Deirochelys have a distinct 

 continued vertebral line, not found in any of the other genera, 

 Chrysemys being peculiar for having a very distinct well-marked 

 pale edge to the dorsal shields, while Deirochelys has a dark spot 

 surrounded by reticulated lines on each shield. 



The variegated species of the genus Trachemys (if the genus 

 ought not to be restricted to those species) and Pseudemys 

 {Ptychemys, Agassiz) have several eye spots, which are often 

 more or less confluent and separated by pale or bright-coloured 

 lines under each dorsal shield, the former genus having a convex 

 horny lower beak, and the latter a flattened lower jaw with a 

 small thin lower beak and broadly webbed toes. 



The genera CalUchelys (of which Emys ornata may be con- 

 sidered the type) and Malaclemys have a single eye spot, sur- 

 rounded by regular concentric rings, under each dorsal shield. 

 The Callichelyes have a hard thin skin on the head, and the 

 centre of the rings approaches the hinder edge of the shield as 

 the shields enlarge. Some specimens of this genus have a pale 

 streak down the centre of the nuchal plates. The Malaclemyes 

 have a soft fleshy skin on the head, the centre of the spot re- 

 mains in the middle of the shields, and the feet of the latter are 

 largely webbed. 



