78 SUPPLEMENT TO THE 



In the costal plates (T. XXVIII A, pi \—pi %) the general form and proportions are 

 sufficiently clearly represented; but it is worthy of special remark that the first (pi 1) 

 has a small quadrate portion marked out on its inner or median border, by the impres- 

 sion of a supplementary median scute, which I have not observed in other Emydians, 

 recent or fossil ; and should that supplementary scute be constant in the present 

 species, such species might be indicated by a detached fossil first costal plate [pi 1). 



In Emydians generally the vertebral scutes are five in number ; the costal scutes 

 are four pairs : collectively, when they have been called " discal " scutes or plates, 

 thirteen is the number. In the present extinct species they were fourteen in number, 

 owing to the division of the first vertebral scute (v l), or to the interposition of a small 

 transversely extended quadrate scute between it and the second vertebral scute (» 2). 

 It will be fortunate if other specimens of the Emys Conybearii should show this cha- 

 racter to be a constant one. 



In Emys Delabeclm the carapace is unluckily mutilated at the part requisite for 

 determining whether the supplementary median scute existed. The second vertebral 

 scute (v 2) in Emys Delabechii is as broad as it is long : in Emys Conybearii the breadth 

 of the same scute is one third greater than its length. A similar difference of propor- 

 tions, with more produced and acute lateral angles, characterises the third vertebral 

 scute of Emys Conybearii as compared with Emys Delabechii. 



The fourth vertebral scute in Emys Conybearii has the front half of its lateral 

 margin wavy or crenate, not a simple sigmoid line, as in Emys Delabechii. The 

 breadth and depth of the scutal impressions are alike in both species. 



In Hydromedusa the number of discal scutes, or those inclosed by the marginal 

 scutes, is fourteen, as in the present fossil ; and, as it appears to me, by a similar 

 transverse division of the first vertebral scute ; only the dividing line crosses the scute 

 more anteriorly, so that the proportions of the front and hind divisions are reversed. 

 Dr. Gray regards the front division as the homologue of the front marginal scutel 

 called "nuchal" in other ChelydidcB and in Emydida, and characterises Hydromedusa 

 as having " the nuchal plate large, placed behind the front marginal plate, like a sixth 

 vertebral ;" but if the carapace of Hydromedusa depressa* be compared with that of 

 Chehdina oblonya,f and Sternothcerus Derbianus,\ the marginal series of scutes will be 

 seen to be similar in number in the two species, and their condition to be essentially 

 that which is defined in the characters of Sternotharus, as " Nuchal plate none."§ 

 The part called " nuchal plate " in Hydromedusa depressa answers to the front part of 

 the first vertebral plate in Sternothoerus and Chehdina : the different position of the 

 dividing line in the Emys Conybearii more strongly marks the true homological character 

 of the first of the median series of discal scutes. I conclude, therefore, that, as in 



* 'Catalogue of Shield Reptiles,' Brit. Mus., 4to, p. 59. 

 f lb., tab. xxvi. % lb., tab. xxii. § lb., p. 51. 



