TiaONTCHOlDEA. ^23 7 



Supcrfam. C. TRIONYCIIOIDEA. 



Amydfe, part., Oppel, Ordn. liept. 1811. 



Trionvcliuidca, Sfa/iiiiii)i, Zoot. A)iiji/i. lS/54. 



Auiydiu, part., Aj/aasiz, Coiitr. Aat. Hid. U. S. i. 1857. 



Trionj'clioidea, Grai/, iSiippl. Cat. S/i. liept. i. 1870. 



Cry])t(:dira, part., Cupe, I'ruc. Anwr. Assoc. Adv. Sc. xix. 1870, p. 23o. 



Peltoclit'lyidiu, Scvlfi/, Q. Jouni. Gevl. iSoc. xxxvi. 18b0, p. 4l'J, 



Dactvlosterua, part., Cope, Pruc. Amcr. Phdos. Hoc. xx, 1882, 



p. 148. 

 Dactylt)plastra, part., Dollo, Bull. Mus. Belg. iv. 188G, p. 91. 

 Diacostoidea, L'aur, Zoul. Anz. 1887, p. 99. 



Neck bending by a sigmoid curve in a vertical plane ; cervical 

 vertebra? without well-developed transverse |)rocesses ; articulation 

 between the last cervical and tlie first dorsal vertebra by tlic 

 zygapophyses only. Mandible with articulary concavities ; outer 

 boi'dcr of tympanic cavity notched ; pterygoids not narrowed pos- 

 teriorly, separated from eacb other, the basispheuoid joining the 

 palatines. Pelvis not anchylosed to the carapace and plastron. 

 Fourth digit with four or more phalanges. Epiplastra separated from 

 the hyoplastra by the yy-shaped cntoplastron. Marginal bones 

 absent or forming an incomplete series, not connected witb the ribs. 



DoESAL Yertueeal Column axd Caeapace. — As in the Cryptodira, 

 the dorsal vertebral column consists of ten vertebrae, the first and last 

 of which are free from the carapace and have small or vestigial free 

 ribs ; the centra are flattened, with a more or less distinct ventral 

 keel. The first dorsal vertebra articulates with the last cervical 

 merely by its praezygapophyses. Eight ribs contribute to the for- 

 mation of the dorsal disk, but in the American species of Trionyx, 

 which have but seven costal plates, the last remains free ; the 

 vertebral attachment of all except the latter is between two centra. 

 All the dorsal j^lates are sculptured, pitted, vermiculate, or granulate. 

 The neurals, the greater part of which are six-sided, with the 

 postero-lateral side the shortest, number generallj' seven or eight ; 

 but the}' are much reduced in number and in size in CycJanorhis, 

 in which, as in some Pleurodira, the costal plates meet on the 

 median line between them. Except in some Trioiri/.v, the last or 

 last two pair of costals join in a median suture. The nuchal is 

 much broader than long and, in Trionyx, Peloclidys, and Chitra, 

 each end overlaps the extremity of the rib of the second vertebra. 

 In Emyda, Cyclanorhis, and CycJodenna, on the contrary, the outer 

 extremities of the nuchal bone are overlapped by the first costal 

 plate. Pygals are absent, and likewise marginals connected with 

 the ribs. Put, in Emyda, the posterior fleshy border of the dorsal 

 disk contains a series of sculjjtured bones which ai)pear to form the 

 continuation of the lateral plastral plates ; these bones are not 



