24 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxii. 



No evidences of serrations on the border behind. Median line appar- 

 ently occupied by a keel of low bosses. 



Surface of the carapace mostly smooth, with the impressions of the 

 shield sutures distinct; areas occupied by costal shields with some 

 longitudinal wrinkles. First neural nearly square; the eighth pentag- 

 onal, with the anterior side very short. The single pygal extremely 

 broad, extending across three marginals. The nuchal is considerably 

 broader than long. Marginals quadrate; the lateral ones each placed 

 opposite the end of a costal. The second and third vertebral shields 

 enormous, extending laterally beyond the middle of the costals, the 

 anterior border of the second vertebral occupying its usual position. 

 Its hinder border thrown back to the hinder border of the third neural. 

 The hinder border of the third vertebral not crossing the fifth neural, 

 the usual position, but thrown backward to near the hinder border of 

 the sixth neural. The fourth neural is thus made very concave in front. 

 Costal shields narrow, not one-half as wide as the contiguous verte- 

 brals, alternate sutures between lateral marginal shields coinciding 

 with sutures of costal shields. 



As to the relationships of this genus we can say nothing certain until 

 we have at least had an opportunity to study the plastron. E^everthe- 

 less its affinities appear to be with the Chelydridse, and we may place it 

 there provisionally. It is of interest to compare the figure on Plate VI 

 with the diagrams of the carapaces of Chelydra and ^lacroclemmys, 

 found in Boulenger's Catalogue of Chelonians.^ The carapaces of all 

 three genera have the same general form; in all, the neural bones are 

 closely similar; in all, the marginals lie opposite the outer ends of the 

 costals, instead of alternating with the latter. In Chelydra and Macro- 

 clemmys, however, there are more or less extensive fontanelles between 

 the costals and marginals. In both, the vertebral shields are of mod- 

 erate width ; in both, the hinder border of the shell is serrated. Length 

 of the carapace, 181 mm ; its width, 118 mm. 



Certain described genera of fossil turtles display vertebral shields 

 as broad, perhaps, as those of the form here described. Among these 

 may be mentioned Sylwochelys, which is figured by Dr. Lydekker.^ In 

 this genus, however, the neural bones are long and narrow. Flesiochelys, 

 a figure of which may be found in Zittel's Handbuch, page 545, also 

 has very broad vertebral shields; but here again the neurals are nar- 

 row. Platychelys^ is also furnished with broad vertebrals, but it is in 

 many respects different from Acherontemys. The neurals are very irreg- 

 ular in form and size; the carapace has a different form and is covered 

 with large bosses. 



It is to be hoped that Mr. Heckman will be able, through the miners 

 in his employ, to secure other specimens of this species, so that the 

 structure of the plastron may be determined. 



1 Pages 22, 25. 



2 Catalogue of the fossil Eeptilia and Amphibia, Pt. 3, p. 188, fig. 43. 

 ^Lydekker, Catalogue of the fossil Reptilia and Amphibia, Pt. 3, p. 217, tig. 48; 



Zittel, Handbuch, III, p. 533, fig. 499. 



