183 PLEtJRODIRA. 



(level npt'd, 1 hough never to such a degree as in the Cryptodiran 

 Uata"-urs ; tliey arc very short in Felotneduaa and Sttrnotharus, 

 and in tlic adult of the latter geuus, in which the front plastral 

 loho is movable, in quite a vestigial condition, as in tlie Crypto- 

 diran genera Eiirt/s and Cistndo. 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. 



Ckuvical VEKTEiiRj\ — The presence of well-developed trans- 

 verse processes, situated behind the vertical of the pnczygapophyses, 

 and the absence of any ginglynioid articulations, distinguish the 

 cervical vcrtebne 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 Fodocnenm, Eudjdura, and Elsci/a, and I have 

 also found them so in Hydraspis hUarii. The mode of articulation 

 of the cervical vertebra) is diagnostic of the families : thus, in the 

 Pelomedusida), the second only is biconvex, the following being 

 concavo-convex ; in the Chelydidtc, the second, third, and fourth 

 are convexo-concave, the fifth biconvex, the sixth concavo-convex, 

 the seventh biconcave, and the eightli biconvex. 



Caudal Vertebrje. — They are proccelous and provided with well- 

 developed transverse processes; the number is usually IG to 18; 

 21 in Fodocnemis exyansa, 25 in Emi/dunc macquarice. Chevron 

 bones are absent. 



Skull. — 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 Fodocnemis. The orbit is bounded by the max- 

 illary, the pra^frontal, the jugal, and (except Cheh/s) the frontal. 

 The prajmaxillaries, wliich are fused to a single bone in Cheli/s 

 alone, are well developed and bound the nasal opening. Nasal 

 bones are distinct from the pra)frontals in all Chelydida) except 

 Chelys *. The frontals and the parietals are distinct, the latter 

 bones being connected with the palate by descending processes. In 

 the Chelydida) the frontals are produced forwards, between the 

 prjcfrontals, to the nasals, or to the nasal opening. A complete 

 bony temporal roof is present only in Fodocnemis, 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 Pelomcdusidte 

 there is a well-developed zygomatic arch, formed by the postfroutal, 

 the jugal, and the quadrato-jugal, the latter element separating the 

 two former from the squamosal and the quadrate. In the Chelydida3 

 the zygomatic arch is absent, but a paricto-squamosal arch is 

 ])rcsent in all genera save Chclodina. In FluUmys, Emydura, 

 Elseyn, and, to a lesser extent, in I/ydraspis, 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, 



* Briilil and Hoffuiann, however, represent them in that geuus. 



