TRIONYCHOIDFA. D3 
Superfam. C. TRIONYCHOIDEA. 
Amydee, part., Oppel, Ordn. Rept. 1811. 
Trionychoidea, Stannius, Zoot. Amph. 1854. 
Amydee, part., Agassiz, Contr. Nat. Hist. U.S. i. 1857. 
Trionychoidea, Gray, Suppl. Cat. Sh. Rept. i. 1870. 
Cryptodira, part., Cope, Proc. Amer. Assoc, Adv. Sc. xix. 1870, p. 235, 
Peltochelyidx, Seeley, Q. Jowrn. Geol. Soc. xxxvi. 1880, p. 412. 
Dactylosterna, part., Cope, Proc. Amer. Philos, Soc. xx. 1882, 
. 145, 
Tactyloplactees part., Dollo, Bull. Mus, Belg. iv. 1886, p. 91. 
Diacostoidea, Baur, Zool. Anz. 1887, p. 99. 
Neck bending by a sigmoid curve in a vertical plane; cervical 
vertebra without well-developed transverse processes ; articulation 
between the last cervical and the first dorsal vertebra by the 
zygapophyses only. Mandible with articulary concavities; outer 
border of tympanic cavity notched ; pterygoids not narrowed pos- 
teriorly, separated from each other, the basisphenoid 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 a-shaped entoplastron. Marginal bones 
absent or forming an incomplete series, not connected with the ribs. 
Dorsat VerresraL Cotumn anp Carapace.—As in the Cryptodira, 
the dorsal vertebral column consists of ten vertebre, 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 preezygapophyses. Eight ribs contribute to the for- 
mation of the dorsal disk, but in the American species of Trionya, 
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 plates are sculptured, pitted, vermiculate, or granulate. 
The neurals, the greater part of which are six-sided, with the 
postero-lateral side the shortest, number generally seven or eight; 
but they are much reduced in number and in size in Cyclanorbis, 
in which, as in some Pleurodira, the costal plates meet on the 
median line between them. Except in some 7’rionyx, the last or 
last two pair of costals join in a median suture. The nuchal is 
much broader than long and, in Vrionyx, Pelochelys, and Chitra, 
each end overlaps the extremity of the rib of the second vertebra. 
In Emyda, Cyclanorbis, and Cycloderma, 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. But, in Emyda, the posterior fleshy border of the dorsal 
disk contains a series of sculptured bones which appear to form the 
continuation of the lateral plastral plates; these bones are not 
