2 MAEINE EEPTLLES OE THE OXFOED CLAY. 



allied to the other members of this family, but is excluded from it by the possession 

 of single-headed cervical ribs. 



Williston* has attempted a definition of this family on the assumption that the 

 genus Brachauchenius from the Cretaceous of Kansas is closely allied to Pliosaurus. 

 His definition is : — " Skull depressed, no parietal crest ; palatines broadly contiguous on 

 the middle line ; pterygoids with a prominent ridge and abutting mandibular process. 

 Neck short, cervical ribs single- or double-headed, all vertebras without infracentral 

 vascular foramina." Such a definition would exclude the type genus from the family. 

 Probably the North-American reptiles corresponding to the Pliosaurs of Europe will 

 be found to constitute a distinct family, in which the characteristics common to the 

 two groups are the consequence of parallel modifications. 



Genus PLIOSAURUS, Owen. 



[Odontography, pt. ii. (1841) p. 282.] 



1873. Liopieurodon, Sauvage, Bull. Soc. Geol. France, [3] vol. i. p. 377. 



Skull relatively large, with an elongated rostrum ; mandibular symphysis moderately 

 long, extending back to about the level of the seventh tooth. Teeth sharp-pointed, 

 with numerous longitudinal ridges of enamel of varying length, usually absent or only 

 slightly developed on the outer side, which in the later (Kimmeridgian) forms may be 

 flattened, so that the crown is roughly trihedral in section. Neck short, consisting of 

 twenty-two or twenty-three vertebras, with short centra and high neural spines. 

 Cervical ribs with double heads, except, perhaps, the last. Dorsal and sacral vertebrae 

 at least twenty-four in number ; caudals at least fifteen, the posterior three or four 

 diminishing in size very rapidly. 



Skull (PI. I. ; text-fig. 1). — The skull in this genus has already been described in some 

 detail in the Quarterly Journal of the Geological Society, 1897, p. 177, and although 

 other specimens have been obtained since that description was written, none are better 

 preserved or add much to our knowledge of this important part of the skeleton. The 

 specimen (R. 2680) there described and figured is, therefore, now taken as the basis for 

 a somewhat more detailed account, and is figured on PI. I. It is unfortunate that this 

 skull was not associated with the mandible or any other part of the skeleton. A second 

 and more imperfect skull (R. 3536) is also referred to — this specimen being of especial 

 importance, because it is associated not only with the mandible, but also with a 

 considerable portion of the rest of the skeleton. 



In its general outline the skull is a greatly elongated triangle, its length being 

 about 2| times its width at the quadrates. The temporal fossae are very large and are 

 separated by a narrow and high sagittal crest. The orbits seem to have been rather 



* Science, n. s. vol. xvii. (1903) p. 9S0. 



