2 PLACODONTIA. 



Skull with a deep temporal arch, apparently consisting of a 

 coalesced upper and lower arcade : ; and the palate completely 

 closed by the union of the pterygoids with one another and with 

 the basisphenoid 2 . No bony floor to the nasal passage, and the 

 posterior nares consequently opening by slits on the surface of the 

 palate. Dentition mainly in the form of pavement-like teeth, 

 occurring both in the alveolar margins of the jaws and also on the 

 surface of the palate ; teeth apparently inserted in very shallow 

 sockets. The skull has a parieto-squamosal (post- temporal) and a 

 postorbital bar. 



Family PLACODONTID^E. 



The whole of the known forms may be included in this family. 

 Skull more or less broad and depressed, with double nares, which 

 may be approximated to the orbits. Upper teeth arranged in an 

 outer or maxillary series of small ones, and an inner or palatine 

 series of large ones ; mandibular teeth in a single row. Anterior 

 teeth, in some cases, of a prehensile type. 



Genus PLACODUS, Agassiz 3 . 



Cranium longer than broad, with a distinct premaxillary rostrum 

 carrying three chisel-like teeth on either side ; premaxillaB separate ; 

 three palatine teeth of polygonal contour ; and four or five maxil- 

 lary teeth. Mandible with produced rostrum containing two chisel- 

 like teeth on either side of the extremity. 



Placodes gigas, Agassiz 4 . 



Syn. Placodus pachygnathus, Owen 5 . 

 Placodus batkygnathus, Owen G . 

 ? Placodus hypsiceps, Meyer 7 . 



The type species. Of large size, with a vaulted cranium, in 

 which there are four comparatively large maxillary teeth on either 

 side. 



1 Shown in the imperfect skull figured in the ' Palacontographica/ vol. xi. 

 pi. xxvii. 



2 This is a character common to the Chelonia and to the typical Notho- 

 sauridcB among the Sauropterygia. 



3 Eecherches sur les Poissons Fossiles, vol. ii, pt. i. p. 15 (1833). 



4 Loc. cit. 



5 Phil. Trans, for 1858, p. 179 (1859). 

 Ibid, p, 182. 



7 ' Pakeontographica,' vol. xi. pt. iv. p. 199 (1863). 



