PLEURODIRA. 189 
which, as in the other Chelonians, is deeper than broad. In Sterno- 
therus and Pelomedusa the basioccipital is completely excluded 
from the occipital condyle. 
The quadrate forms a complete frame to the tympanum, rarely 
with a very small interruption posteriorly, but a part of the outer 
and the whole of the middle ear-chamber remain open, the stapes 
being entirely visible from behind. The quadrate presents a large 
concavity for the reception of the single condyle of the mandible, 
and the pterygoids extend to the very edge of that concavity. The 
quadrate forms a suture with the basisphenoid, except in Chelys, in 
which it is separated from the latter by the prootic ; in Podocnemis 
it joins both the basisphenoid and the basioccipital. 
The single yomer is well developed and separates the palatines 
in the Chelydidee, rudimentary or absent in the Pelomeduside, 
in which the palatines form a median suture. In Chelys and 
Chelodina, but not in the other Pleurodira, the prefrontals join 
the palatines; these genera thus differ by the presence of a bony 
septum between the orbital and nasal cavities. The pterygoids 
are extremely broad, join on the median line, and their outer borders 
expand into thin lateral wings, which in Podocnemis extend pos- 
teriorly as palatal laminze distinct from the processes which join 
the quadrates. A more or less developed, rolled up lateral process 
is present, which borders a deep canal leading into the orbital 
cavity. As pointed out by Rutimeyer, the presence of this canal 
isin correlation with the extent to which a postorbital bony septum, 
formed by the postfrontal, the Jugal, and the palatine, is developed ; 
the more complete the separation of the orbital cavity from the 
temple, the larger the canal; Chelys stands, in this respect, at the 
lower end of the series, the arrangement not being carried out much 
further than in the most thoroughly aquatic Testudinide. In none 
of the Pleurodira do the pterygoids extend posteriorly beyond the 
quadrates ; in Chelys, Emydura, and Elseya they join the maxillary 
anteriorly, whilst in the other genera they are separated from the 
latter by the palatine and jugal. 
The mandible articulates with the skull by a large single condyle, 
in which the posterior elements are usually completely fused. A 
symphysial suture is present in the Chelydids, at least in young 
individuals. 
Hyorm Arcu.—The hyoid is much developed in the Chelydide, 
especially in Chelys, Hydromedusa, Chelodina, and Hydraspis. The 
body is fully ossified, elongate, and hollowed out as a canal for the 
trachea ; in the four above-named genera, there are four ossifications 
anteriorly, with a membranous space between them and the pos- 
terior, elongate, canal-like bone. Two pairs of ossified cornua are 
present, the anterior the strongest. Chelys has a pair of very large 
bony entoglossals, forming together a ‘Y~-shaped figure ; entoglossal 
bones are also present in Hydromedusa, but much smaller and sub- 
circular in shape; they are absent in Chelodina and the other genera. 
In the Pelomedusidée the hyoid is comparatively feebly developed, 
more as in the Testudinide. The body is cartilaginous and asingle 
pair of ossified cornua are present, the posterior being cartilaginous. 
