2 MARINE REPTILES OF THE OXFORD CLAY. 



Vertebral centra amphicoeloiis and usually verj^ short. A varying number of the 

 anterior ribs with a double articulation with the vertebrae. No sacrum. 



In the shoulder-girdle, distinct clavicles and an interclavicle, which is usually 

 T-shaped (triangular in Leptocheirus, Merriam *). Pelvis, at least in the later forms, 

 much reduced, and the ilium not united with the vertebral column. Limbs modified 

 to form paddles, and the fore limb, as a rule, considerably the larger (not in the Triassic 

 ^enws Toretocnemus, Merriam f); digits varying in number from three to six and 

 consisting of numerous phalanges. 



Eange from Trias to Lower Chalk. 



Family OPHTHALMOSAUEID.E. 



Specialised Tchthyosaurs in which the orbit is extremely large and the dentition 

 reduced in the adult to a number of small teeth, which are loosely set in the anterior 

 half of the jaws only. The humerus articulates distally with three bones, and the 

 fore paddle is much larger than the posterior, which is greatly reduced; the pubes and 

 ischium are fused together. 



Middle and Upper Jurassic of Europe and North ^America, with doubtful repre- 

 sentatives in the Upper Greensand of England. 



Genus OPHTHALMOSAURUS, Seelev. 

 [Quart. Journ. Geol. Soc. vol. xxx. (1874) jj. IJUi).] 



1879. Sauranodon, 0. C . Marsli, Amer. Journ. Sci. [3] vol. xvii. p. 85. (Name previously 



employed by Jourdan for a Rliynchocephaliau |.) 



1880. Baptanodon, 0. C. Marsli, Amer. Journ. Sci. [.3] vol. xix. p. 491. 

 1902. Microdontosaiints, C. "\V. Gilniore, Science, n. s. vol. x^'i. p. 914. 



Orbit very large ; teeth small, confined to the anterior half of the jaws, at least in 

 the adult. Clavicles uniting in a complex suture (sometimes fused) and embracing 

 the anterior bar of the interclavicle ; coracoid normally without posterior notch. 

 Humerus with strong trochanteric ridge and articulating distally with three elements. 

 Pelvis greatly reduced ; ischium and pubis normally fused with one another at the 

 ends and enclosmg between them a small foramen. Hind limb very small, femur 

 articulating distally with two elements only. 



Middle and Upper Jurassic. 



This genus was estabhshed {loc. cif. supra) in 1874 by Prof. H. G. Seeley for the 



* " New Ichthyosauria from the Upper Triassic of CaHfomia," Bull. Geol. Dept. Univ. California, vol. iii. 

 (1903) p. 253. 

 t Tom. cit. p. 259. 

 X Quoted by Gervais in Comptes Rendus Ac. Sci. Paris, vol. Ixxiii. (1S71) p. 605, from Jourdan's MS. 



