ANOMODOXTIA 235 



this modified form of Enaliosaur are peculiar to the Oxfordian 

 and Kimmeridgian divisions of the upper oolitic system. They 

 have been discovered in these beds in Russia {Pliosaurus 

 Worinskii and Spondylosaurus of Fischer), as well as in those 

 counties of England where the Kimmeridge and Oxford clays 

 have been deposited. 



Genus Polyptychodon. — A genus represented by species 

 equalling in size those of Pliosaurus. The teeth have a strong 

 conical crown with a sub-circular transverse section, and the 

 longitudinal ridges of the enamel are set close all round the 

 crown, whence the name of the genus, signifying "many- 

 ridged tooth ;" they may be distinguished from the teeth of 

 Mosasaurus or Pliosaurus by the absence of the smooth almost 

 flattened facet of the crown, which, in those genera, is divided 

 by two strong ridges from the rest of the crown. The teeth 

 are implanted in distinct sockets, as in Plesiosaurus. The 

 vertebras found in the same strata, corresponding in size with 

 the teeth, present the plesiosauroid type. Bones of a large 

 paddle or natatory limb, from the chalk of Kent, may also 

 belong to Polyptychodon. A portion of the cranium of Polyp- 

 tychodon interruptus, from the chalk, shows the " foramen 

 parietale," and a plesiosauroid type of temporal fossae. 



Remains of Pohjptychodon have hitherto been met with 

 only in the cretaceous formations : in the green-sand of Kent 

 and Cambridge, also at Kursk, in Russia, and in the chalk of 

 Kent and Sussex. 



Order 5. — Anomodontia. 



Teeth wanting, or limited to a single maxillary pair, having 

 the form or proportions of tusks : a " foramen parietale ;" 

 two nostrils ; tympanic pedicle fixed ; vertebras bicon- 

 cave ; trunk-ribs long and curved, the anterior ones 

 with a bifurcate head ; sacrum of more than two 

 vertebrae. Limits ambulatory. 



