PLEURODIRA. 189 



which, as in the other Chelonians, is deeper than broad. In Slcrno- 

 tJuenis and Pclomedasa the basioccipital is completely excluded 

 from the occipital condyle. 



The quadrate Ibi-ms a complete frame to the tympanum, rarely 

 with a very small interruption posteriorly, but a pai't of tho outer 

 and the whole of the middle ear-chamber remain oj)cn, the stapes 

 being entirely visible from behind. The quadrate presents a large 

 concavity for the reception of the single condyle of the mandil)le, 

 and the pterygoids extend to the very edge of that concavity. The 

 quadrate forms a suture with the basisi)henoid, except in Chi'li/s, in 

 which it is separated from the latter bj' the prootic ; in Podocnemis 

 it joins both the basisphenoid and the basioccipital. 



The single vomer is well developed and separates the palatines 

 in the Chelydidie, rudimentary or absent in the Pelomedusidae, 

 in which the palatines form a median suture. In Chclys and 

 Cheloditia, but not in the other Pleurodira, the pnefrontals join 

 the palatines ; these genera thus diifer by the presence of a bony 

 septum between the orbital and nasal cavities. Tlie j)terygoids 

 are extremely broad, join on the median line, and their outer borders 

 expand into thin lateral wings, which in Pudocncmis extend i)os- 

 teriorly as palatal laminte distinct from the processes which join 

 the quadrates. A moi'e or less developed, rolled u]) lateral process 

 is present, which borders a deep canal leading into the orbital 

 cavity. As pointed out by lUitimeyer, the presence of this canal 

 is in correlation with the extent to which a postorbital bony septum, 

 formed by the jiostfrontal, the jugal, and the palatine, is developed ; 

 the more complete the separation of the orbital cavity from the 

 temple, the larger the canal ; CheJys 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 TcstudinidcC. In none 

 of the Pleurodira do the pterygoids extend posteriorly beyond the 

 quadrates ; in Cheli/s, Emydura, and Eheya they join the maxillary 

 anteriorly, whilst in the other genera they are separated from tho 

 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 Chelydid*, at least in j'oung 

 individuals. 



Hyoid Akch. — The hyoid is much developed in the Chelydidoe, 

 especially in Chehjs, Hydromedttsa, Clulodina, and Hydrastis. 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 Pelomedusidie the hyoid is comparatively feebly developed, 

 more as in the Testudiuidge. The body is cartilaginous and a single 

 pair of ossified cornua are present, the posterior being cartilaginous 



