Middle Trias to Upper Cretaceous. Families : Mixosauridae, Ichthyosauridae.
Order Chelonia. Reptiles in which the trunk is enclosed in a shell built up from a series of dermal bones, with which the neural arches and ribs become continuous. The limb girdles are unique, in that they lie entirely within the ribs. Skull without any temporal vacuity, but the continuous sheet of bone which, in the primitive forms, overlies the temporal muscles may be emarginated either from the back or from below, or from both, so that in extreme cases the squamosal may be left without con nection with any other bones of the skull roof. The powerful vertically-placed quadrate is then only supported by its abutment in the pro-otic opisthotic and pterygoid.
Postfrontals and lachrimals are always, nasals usually absent, and the external nares are confluent. Except in Triassochelys, the jaws are toothless, and they always support a horny beak. There is often a small secondary palate, not homologous with that of mammals or crocodiles, formed by extensions of the palatines and prevomers. The eight cervical vertebrae are so formed that the neck is flexible, bending into a vertical loop in Cryptodeira, and into a horizontal S in Pleurodeira.
The dorsal vertebrae are ten in number, the first being free, or nearly so, from the shell, whilst the rest are fixed immovably by their attachment to the neural plates of the carapace; the two sacral vertebrae are similarly attached. The posterior dorsal vertebrae are peculiar, in that each of their neural arches rests on two centra.
The shoulder girdle consists of a scapula, whose acromian process is produced into a long rod lying horizontally, and approaching its fellow in all forms except Triassochelys, and a corticoid which form a curious pedunculate glenoid cavity. Cleithra are present only in Triassochelys. Clavicles and an inter clavicle are entirely detached from the shoulder girdle and form part of the plastron.
The ilium usually articulates, not with the sacral ribs, but with the carapace; the pubis and ischium are lacertilian-like. The limbs are much modified, in order to reach the girdles which lie within the shell, and to allow of their withdrawal in the more primitive forms. The fifth metatarsal is hook shaped. Both feet
are pentadactyl, the phalangeal formula never exceeding 2, 3, 3, 3, 3, and being sometimes reduced to 2, 2, 2, 2, o.
The shell, in its fullest development, consists of a dorsal carapace, built up from a median row of nuchal, preneural, eight neural and two suprapygal bones, and lateral rows, each of eight costals, articulating with the neurals, a variable development of supra marginals may occur secondarily; in their absence the costals articulate with a continuous chain of marginals which connect carapace and plastron.
The plastron consists of three plates anteriorly, the epiplastra which are clavicles, and an entoplastron, the interclavicle; a pair of hyoplastra, two pairs of mesoplastra, one pair of hypoplastra and one of xiphiplastra. The neural bones are co-ossified with the neural spines of the dorsal vertebrae, the costals with the ribs of the second to ninth dorsals.
Sub-order Amphichelydia. Chelonia with no power of com pletely withdrawing the head within the shell, mesoplastra pres ent, pelvic girdle not fused with the plastron. Families not yet discriminated. Middle Trias to Eocene.
Sub-order Pleurodeira. Chelonia which withdraw the head sideways. Mesoplastra usually present, pelvic girdle fused with the carapace, and usually with the plastron. Families: Pelome dusidae, Chelyidae, Miolanidae, Plesiochelyidae. Jurassic to Recent.
Sub-order Cryptodeira. Chelonia which withdraw the head vertically. Mesoplastra absent, pelvic girdle never fused with the plastron. Families : Thalassemydidae, Chelydridae, Testudini dee, Cinosteriidae, Platysternidae, Cheloniidae, Protostegidae, Dermochelyidae, Dermatemydidae, Trionychiida,e. Jurassic to Recent.
There is a considerable number of small Palaeozoic reptiles which do not fall into any of the 19 orders defined above. Of these the more important are : Eosaurus from the Coal Meas ures of the United States, which may be Cotylosaurian; Euno tosaurus from the Upper Permian of South Africa, which may be an ancestor of the Chelonia ; Broomia from the Upper Permian of South Africa, which may be an ancestral lizard; and Araeos celis, from the Lower Permian of Texas, which has also been regarded as a lizard ancestor.