xu 



MAEINE EEPTILES OE THE OXFOED CLAY. 



Triasslc Ichtliyosauria. 



1. The orbits are relatively small and the tem- 



poral bar behind them broad. 



2. The maxilla relatively large and the pre- 



maxilla correspondingly smaller. 



3. Teeth set in distinct sockets and those in the 



posterior part of the jaws often differing 

 in form from those in front. 



4. The anterior cervical vertebra; separate from 



one another. 



5. The neural spines thick and sometimes 



circular in section. 



6. The zygapophyses of opposite sides separate 



from one another. 



7. The terminal portion of the caudal series 



of vertebrse only slightly bent down, the 

 caudal fin being comparatively small and 

 in some cases supported by the elongated 

 neural spines (and sometimes chevrons). 



8. Hind limbs larger in proportion to the fore 



limbs than in the later forms, the reduction 

 of the hind limb, however, already making 

 considerable progress in some species, 

 e. g. Mixosaurus nordenskioldi. 



9. The epipodial bones elongated, with traces 



of a shaft. 



10. The pelvic bones heavy, the ischium and 



pubis expanded and never fused with one 

 another. 



Oplitlialmosaurits. 



1. The orbits very large and the temporal bar 



behind them greatly reduced in width. 



2. The maxilla small and edentulous and the 



premaxilla relatively very large. 



3. The teeth, when present, small and loosely 



fixed in a continuous groove, the anterior 

 and posterior teeth of the same form (as 

 in Jurassic Ichthyosaurs which possess 

 maxillary teeth). 



4. The two anterior cervical centra (atlas and 



axis) fused with one another. 



5. The neural spines broad and strongly com- 



pressed laterally. 



6. The zygapophyses of opposite sides, in most 



of the vertebrse, in the same plane and 

 imited in the middle line. 



7. The terminal portion of the caudal region of 



the vertebral column sharply bent down, 

 and, notwithstanding its large size, the 

 caudal fin not supported by the neural 

 spines or chevrons, which are much 

 reduced. 



8. Fore limbs much larger than the hind limbs. 



9. The epipodial bones shortened, with no trace 



of a shaft. 



10. Pelvic bones small, the ischium and pubis 



fused with one another at both ends. 



In the Jurassic Ichthyosaurs other than Ojjlithahnosaurus, the differences from the 

 Triassic types above enumerated are, of course, nearly equally well marked; but those 

 numbered 1, 2, 8, and 10 are especially well illustrated by the Oxford Clay type. 



As to the origin of Ophthalmosaurus there is no certainty, but probably it was 

 derived from one of the " latipinnate " group of Ichthyosaurs. There is nothing in 

 the structure of the skull that is opposed to this suggestion, and the arrangement of 

 the bones in tlie paddles seems rather to support it. It is unfortunate that no 

 specimens of the paddles have been collected with the bones in an undisturbed 



