ORDER SAUROPTERYGIA. IO67 



either one or two temporal arcades. A parietal foramen is present 

 at least in the young. In all cases ossifications are developed upon 

 the ventral aspect of the body, either in the form of abdominal ribs, 

 or of a plastron ; but there are none in the sclerotic of the eye. 

 The sacral ribs are connected with the vertebrae by upper and lower 

 articulations ; and when chevrons are present they are mainly or ex- 

 clusively attached to the hinder borders of the caudal centra. A 

 precoracoid anchylosing to the scapula may be present in the pec- 

 toral girdle ; and in the pelvis the pubis and ischium have expanded 

 and flattened ventral surfaces, and the obturator foramen may be 

 completed by the union of the ischium with the pubis of the same 

 side. There is, moreover, a considerable structural resemblance 

 between the limb-bones of the more generalised forms of the two 

 orders, these bones always having terminal epiphyses ; and the 

 tarsus in both may be of a very primitive type. The humerus may 

 have either an entepicondylar (ulnar) foramen and an ectepicondylar 

 (radial) groove, or only the latter, or may be devoid of both. The 

 ribs never have uncinate processes. 



We are still very much in the dark as to the origin of these two 

 orders, although the Sauropterygia can be traced back to a form 

 presenting several Amphibian features, which appears to have been 

 closely allied to the primitive Rhynchocephalians. From the dis- 

 appearance of numerous segments in the vertebral column of the 

 Chelonia during development, Professor Parker has suggested that 

 this order has originated from a type allied to the Sauropterygia ; 

 and their plastron is almost certainly derived from, or developed 

 upon, the abdominal ribs of a form allied either to the Rhyncho- 

 cephalia or to Mesosaurus. 



Order II. Sauropterygia. — In this extinct order the body was 

 devoid of any exoskeleton, while the neck was more or less elon- 

 gated, and the tail short. In the skull there is only the superior 

 temporal arcade ; the narial apertures are lateral and more or less 

 approximated to the orbits ; the premaxillae are very large ; and there 

 is a well-developed parietal foramen in the adult. The prefrontal 

 remains distinct ; the postorbital may be separate from the post- 

 frontal ; typically there is a transverse bone ; and the symphysis of 

 the mandible is united by suture. The teeth, which are implanted 

 in distinct sockets and confined to the margins of the jaws, 1 have 

 curved sharp crowns, with fluted enamel. Each rib articulates to a 

 single vertebra, and in the cervical region the costal facets, which 

 may be either single or double, are situated entirely on the centrum, 

 and generally are not prominent. The vertebrae are amphiccelous ; 

 and the neuro-central suture may be either persistent throughout 



1 Assuming that the Placodontia are distinct from this order. 



