ANOMODONTIA. 255 



teeth are not uncommon : remains of allied species (Pliosaurus 

 Worinskii and Spondylosaurus of Fischer), have been dis- 

 covered in equivalent beds in Eussia. 



Genus Polyptychodon. — This is 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 surface, in those genera, is 

 divided by two longitudinal 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 

 Polyptychodon interruptus, from the chalk, shews the " fora- 

 men parietale," and a plesiosauroid type of temporal fossae. 



Kemains of Polyptychodon have hitherto been met with 

 only in the cretaceous formations : in the upper green-sand 

 of Kent and Cambridge, in the Neocomian at Kursk, in 

 Eussia, and in the chalk of Kent and Sussex. 



The Sauropterygian type attained its maximum dimen- 

 sions under the last two generic forms, at the close of the 

 great mezozoic epoch, when the entire order had passed away. 



Order 5. — Anomodontia. 



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

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

 two external nostrils ; tympanic pedicle fixed ; vertebras 

 biconcave ; trunk-ribs long and curved, the anterior 

 ones with a bifurcate head ; sacrum of more than two 

 vertebrae. Limbs ambulatory. 



