1838 PLEURODIRA. 
developed, though never to such a degree as in the Cryptodiran 
Batagurs; they are very short in Pelomedusa and Sternotherus, 
and in the adult of the latter genus, in which the front plastral 
lobe is movable, in quite a vestigial condition, as in the Crypto- 
diran genera Hmys and Cistudo. A more or less developed notch, 
or canal, is present in the axillary and inguinal buttresses of the 
plastron, at their junction with the marginals. 
Crervican Verresr®.—The presence of well-developed trans- 
verse processes, situated behind the vertical of the preezygapophyses, 
and the absence of any ginglymoid articulations, distinguish the 
cervical vertebree of the Pleurodira. It is also very frequently 
the case that the elements of the first vertebra unite into a single 
bone ; the hypapophysis and the centrum (odontoid process) remain, 
however, distinct in Podocnemis, Emydura, and Hlseya, and I have 
also found them so in Hydvraspis hilarii. The mode of articulation 
of the cervical vertebra is diagnostic of the families: thus, in the 
Pelomeduside, the second only is biconvex, the following being 
concayo-convex ; in the Chelydidw, the second, third, and fourth 
are convexo-concaye, the fifth biconvex, the sixth concayo-convex, 
the seventh biconcave, and the eighth biconvex. 
Caupat Verresrx.—They are proceelous and provided with well- 
developed transverse processes; the number is usually 16 to 18; 
21 in Podocnemis expansa, 25 in Emydura macquarie. Chevron 
bones are absent. 
Sxuti.—The skull is much depressed, most so in Chelys, the extra- 
ordinary flatness of which gives it the appearance of having been 
crushed, least in Podocnemis. The orbit is bounded by the max- 
illary, the prefrontal, the jugal, and (except Chelys) the frontal. 
The premaxillaries, which are fused to a single bone in Chelys 
alone, are well developed and bound the nasal opening. Nasal 
bones are distinct from the prefrontals in all Chelydide except 
Chelys *. The frontals and the parietals are distinct, the latter 
bones being connected with the palate by descending processes. In 
the Chelydide the frontals are produced forwards, between the 
preefrontals, to the nasals, or to the nasal opening. A complete 
bony temporal roof is present only in Podocnemis, and its constitu- 
tion differs greatly from that obtained in the Cryptodira; the post- 
frontal is comparatively small, and the parietal forms a long suture 
with the jugal and the quadratojugal. In the other Pelomeduside 
there is a well-developed zygomatic arch, formed by the postfrontal, 
the jugal, and the quadrato-jugal, the latter element separating the 
two former from the squamosal and the quadrate. In the Chelydide 
the zygomatic arch is absent, but a parieto-squamosal arch is 
present in all genera save Chelodina. In Platemys, Emydura, 
Lilseya, and, to a lesser extent, in Hydraspis, the parietals expand 
into a supratemporal roof, the sides of the temples remaining, how- 
ever, unprotected. 
The supraoccipital is produced posteriorly beyond the parietals, 
and, together with the exoccipitals, bounds the foramen magnum, 
* Brihl and Hoffinann, however, represent them in that genus. 
