190 EEPTILIA class in 



of the pterygoids with the basisphenoid, and frequently also with one another. Narial 

 opening single and terminal. No sternum ; ribs single-headed ; two sacral vertebrae. 

 Pules and ischia forming symphyses. Humerus without entepicondylar foramen. 

 Pentadactyle walking limbs or paddles. 



Chelonians form an extremely homogeneous and narrowly circumscribed 

 group, their peculiar organisation separating them widely from all other 

 reptiles, and their origin and phylogeny being very obscure. A relationship 

 with the Triassic Dicynodonts is perhaps indicated by the skull, which is 

 strikingly similar ; and certain features of the palate and pectoral girdle are 

 suggestive of the Sauropterygia, Rhynchocephalia, and Labyrinthodont Stego- 

 cephalians. Chelonians first make their appearance in the Upper Keuper of 

 Southern Germany (Proganochelys) and exhibit all the typical characters of the 

 order ; nor do they undergo any noteworthy modifications in structure during 

 all their subsequent history. 



Shell. — The most distinctive character of the group is the investment of 

 the body in a more or less rigid shell or box, which is composed partly of the 

 modified neural spines of the dorsal vertebrae, and partly of dermal ossifica- 

 tions more or less intimately united with the former. Into this capsule the 

 limbs, tail, and usually also the neck and head are capable of being retracted. 



In most forms both the dorsal shell or carapace, and the ventral or plastron, 

 are superficially covered with a leathery or corneous epidermal layer, which is 

 divided by indented sutures into a few large scutes or shields. These epi- 

 dermal shields are arranged quite independently of the subjacent osseous plates, 

 and are wanting only in Trionychoidea and Dermochelys. They afford valuable 

 diagnostic characters amongst recent forms, but as a rule are completely 

 destroyed by the fossilisation process, their sutures being merely indicated by 

 shallow sulci. On the carapace they form a median dorsal and a paired lateral 

 series, there being usually five unpaired or vertebral shields, and four or five 

 pairs of costal shields (Figs. 305, 306). Round the periphery there is also a 

 series of about twenty-four marginals (marginalia), the anterior of which 

 is called the nuchal, and the posterior, sometimes double, the caudal or " supra- 

 caudal " shield. Five or six pairs of epidermal shields are present in the 

 plastron, but there is no median series. The anterior pair is designated as 

 gulars, following which in the order named are the numerals, pectorals, 

 abdominals, femorals, and anals. Occasionally the gulars are separated in 

 front by a single or double intergular shield. 



The osseous plates of the carapace (Fig. 297) are formed partly by lateral 

 expansions of the spinous processes of eight dorsal vertebrae (second to ninth), 

 and of the ribs belonging to these vertebrae ; and in still greater part by 

 dermal ossifications overlying the ribs and joining the expanded spinous 

 processes on either side of the median line. The plates of the carapace thus 

 differ in number and arrangement from the epidermal shields, inasmuch as 

 they coincide with the vertebrae and ribs. The latter retain their individuality 

 more or less distinctly on the visceral side of the costal bones, and extend 

 across the vacuities when the carapace is incomplete. 



The median series of eight bony plates in the carapace are called neurals or 

 " vertebrals " ; and the lateral pieces lying superjacent to the ribs, which are 

 firmly united to one another and to the neurals by suture, are termed pleurals 

 (or " costals "). Sometimes not all of the neural bones are developed, and among 

 the recent Pleurodira of Australia they are wanting altogether. In front of 



