TRIONYCHID^). 7 



TRIOJXTYX, Geoffrey 1 . 



Syn. Aspidonectes, Wagler 2 . 

 ? Axestus, Cope 3 . 

 Plastomenus, Cope *. 



The type genus. Skull broad or narrow, with the orbit generally 

 nearer to the temporal fossa than to the nares, and the postorbital 

 bar narrower than the orbit. Usually 7 or 8 neurals. Hypo- 

 plastral with two digitations at postero-external angle 5 . Humerus 

 without channel above ectepicondylar groove. 



The generic synonymy of the recent forms is given by Boulenger, 

 ' Catalogue of Chelonians, &c.' p. 242 (1889). 



The forms from the Eocene and Upper Cretaceous of America 

 described under the name of Plastomenus are stated to differ c by 

 the stouter and more complete ossification of the hyo- and hypo- 

 plastrals, in which the buttresses for the outer digitations are 

 stouter. In the hypoplastral the inguinal border is sharp, and the 

 outer portion of the articular surface for the hyoplastral is con- 

 siderably thicker. These features, which apparently occur in some 

 of the undermentioned forms, do not seem to be of generic value. 

 Further evidence is also required to prove the distinctness of 

 Axestus, since the smooth plastron on which its separation is based 

 occurs in some existing species of Trionyx. 



The existing American forms have but seven costals, but in all 

 the fossil species, both from the Old and New World, in which the 

 entire carapace is known there are not less than eight of these bones. 



Group A. 

 Tivo short neurals between the first pair of costals ; eighth costals 

 meeting in the middle line behind the last neural, ivhich, if the tivo 

 short ones between the first costals be reckoned as a duplication of the 

 normal first, will be the eighth. Symphysis of mandible without oral 

 ridge. 



1 Ann. du Museum, vol. xiv. p. 1 (1809). 



2 Syst. Amphib. p. 134 (1830). 



3 Proc. Amer. Phil. Soc. vol. xii. p. 462 (1873). 



4 Sixth Ann. Eep. U. S. Geol. Surv. Terrs, p. 617 (1873). 



5 In one example of T. hururn in the Museum there are three of these 

 digitations on one side. 



6 Vide Cope, op. cit. and Eep. U. S. Geol. Surv. Terrs, vol. iii. p. 124 (1884). 



