PLEURODIRA. 187 
Superfam. B. PLEURODIRA. 
Emydea monimopelyca, Stannius, Zoot. Amph. 1854. 
Pleuroderes, Gray, Suppl. Cat. Sh. Rept. i. 1870. 
Pleurodira, Cope, Proc. Amer. Assoc, Adv. Se. xix. 1870, p. 235. 
Peltochelyidi, part., Seeley, Q. Journ. Geol, Soc. xxxvi. 1880, p. 412. 
Paradiacostoidea, part., Baw, Zool. Anz. 1887, p. 99. 
Neck bending laterally ; cervical vertebrae with strong transverse 
processes, the cup-and-ball articulation single throughout ; centrum 
of the last cervical articulating with the centrum of the first dorsal. 
Mandible with an articulary condyle fitting into a concavity of the 
quadrate ; outer border of tympanic cavity completly encircled by the 
quadrate ; pterygoids very broad throughout, forming wing-like 
lateral expansions, and in contact on the median line. Pelvis 
anchylosed to the carapace and plastron. Digits with not more 
than three phalanges. Fipiplastra in contact with hyoplastra ; 
entoplastron oval or rhomboidal. A complete series of marginal 
bones connected with the ribs. 
Dorsan Verreprat Cotumn anp Carapacr.—The number of 
vertebrae between the last cervical and the first caudal is ten or 
eleven in all recent Pleurodira, instead of twelve as in all Cryptodira ; 
the reduction being due to the absence of true sacrals or to the 
presence of but a single one, although the last or the last two dorsals 
may assume, in the adult, the character of sacrals. The first dorsal 
centrum is proportionately more elongate than in the Cryptodira, and 
may equal the second in length. The centra are more or less com- 
pressed, except in Chelys, where they much expand towards each 
end for union with the proximally dilated extremities of the ribs. 
In the latter genus, the attachment of all the ribs except the first 
and last two is intervertebral; in the other genera, this is the case 
only with the second to fourth or fifth inclusively. 
The nuchal is usually as long as broad, or longer than broad. 
The neural series rarely (Sternothwrus derbianus) contains more 
than seven plates (Chelys, Hydraspis, Hydromedusa, Pelomeduside 
generally); there is a pretty complete gradation between such a 
series and their total absence (Chelodina, Platemys, Emydura, 
Elseya) : if present, these plates are hexagonal, and short-sided in 
front. Costal plates normally number eight pairs ; but in a speci- 
men of Pelomedusa I find nine pairs, and in another nine plates on 
one side and the normal number on the other. The carapace is 
fully ossified in all adult Pleurodira, and the marginals number 
twenty-three, except in Carettochelys, which has only twenty-one. 
A single pygal is present, and in none of the recent forms does it 
come in contact with the neurals. 
Prasrron.—The nine usual bones compose the plastron in the 
Chelydide ; in the Pelomeduside the number is increased to eleven, 
the additional elements being the mesoplastr a, situated between the 
hyo- and hypoplastra. The plastron joins the carapace by suture, 
and the axillary and inguinal buttresses are usually strongly 
