1066 CLASS REPTILIA. 



arcade, and a postorbital bar. The palatal teeth (rig. 986) resemble 

 paving-stones, and were probably adapted for crushing hard sub- 

 stances like the shells of Molluscs. In the upper jaw the teeth 

 are arranged in an outer or maxillary series of small ones, and 

 an inner or palatine series of larger ones ; all being implanted in 

 shallow sockets and replaced by vertical successors. In the mandible 

 there is but one row of teeth. The number of palatal teeth varies in 

 the different forms ; and there are also modifications in the size and 

 contour of these teeth, which aid in affording generic and specific 

 characters. The premaxillary teeth may be of a more or less 

 prehensile type. Till the vertebrae and limb-bones are known the 

 position of these forms must remain uncertain ; but it may be 

 remarked that all the known limb-bones from the Muschelkalk, 

 except those of Dinosauria, appear to be of a Sauropterygian type. 



In the typical genus Placodus the skull is comparatively narrow, 

 and has a long rostrum produced considerably in advance of the 

 nares. The palatal teeth (fig. 986) have polygonal crowns, those of 

 the palatine series being three in number on either side, and closely 

 approximated ; while the three premaxillary teeth are more or less 

 chisel-like, and are separated by an interval from those on the palate. 

 The mandible has a long symphysis, and two pairs of cutting-teeth. 

 The maxillary teeth may be either four or five on either side. 

 Cyamodus is readily distinguished by the great width and shortness 

 of the cranium, which has no distinct rostrum, with the nares placed 

 at the muzzle and the premaxillae fused together. The palatal teeth 

 have rounded crowns, the crown of the last palatine being very 

 large ; there may be either two palatine and three maxillary, or two 

 maxillary and three palatine teeth. There were but two pairs of 

 premaxillary teeth, which are not chisel-like. In the lower jaw the 

 symphysis was triangular and comparatively short, and was probably 

 devoid of teeth. 



Recently Dr Giirich has proposed the name Pleurodus for an 

 allied form from the Muschelkalk of Silesia, but since this term is 

 preoccupied for a Crocodilian genus it will have to be changed. 



Synaptosaurian Branch. — According to Dr Baur's scheme of 

 classification this branch comprises the orders Sauropterygia and 

 Chelonia ; although Professor Cope and Mr Boulenger would also 

 include the Rhynchocephalia. The typical Proganosauria of Dr 

 Baur may be merged in the Sauropterygia. Although the Sauro- 

 pterygia and Chelonia present many characters in common, yet it is 

 not easy to give a definition of this branch. In all, however, the 

 quadrate is firmly united to the skull ; and all, or nearly all, of the 

 dorsal ribs articulate with the vertebrae by single heads. As a gene- 

 ral rule the palate is more or less completely closed, the pterygoids 

 generally extending forwards to join the vomers. There may be 



