SAUROPTERYGIA. 253 



muschelkalk Saurians, which are as closely allied to Notho- 

 saurus as Pliosaurus is to Plesiosaurus, may have presented 

 analogous modifications in the number and proportions of the 

 cervical vertebrae. It is hardly possible to contemplate the 

 broad and short-snouted skull of the Simosaurus, with its 

 proportionally large teeth, without inferring that such a head 

 must have been supported by a shorter and more powerful 

 neck than that which bore the long and slender head of the 

 Nothosaurus or Pistosaurus. The like inference is more 

 strongly impressed upon the mind by the skull of the Placodus, 

 still shorter and broader than that of Simosaurus, and with 

 vastly larger teeth, of a shape indicative of their adaptation to 

 crushing molluscous or crustaceous shells. 



Neither the proportions and armature of the skull of 

 Placodus, nor the mode of obtaining the food indicated by its 

 cranial and dental characters, permit the supposition that the 

 head was supported by other than a comparatively short and 

 strong neck. Yet the composition of the skull, its proportions, 

 cavities, and other light-giving anatomical characters, all be- 

 speak the close essential relationship of Placodus to Simosaurus 

 and other so-called " macrotrachelian " reptiles of the mus- 

 chelkalk beds. The fin-like modification of the limbs is a 

 better ordinal distinction than the number of vertebrae in any 

 particular region of the spine. But no single character suffices 

 to make known a natural group ; and those who would retain 

 the term Enaliosauria for the large extinct natatory group of 

 saurian reptiles, should bear in mind the essential distinctness 

 of the orders Sauropterygii and Ichthyopterygii, typified 

 by the Ichthyosaurus and Plesiosaurus respectively. 



The generic characters of Pliosaurus are given by the 

 teeth and the cervical vertebrae. As compared with those of 

 Plesiosaurus, the teeth are thicker in proportion to their length, 

 are subtrihedral in transverse section, with one side flattened, 

 and bounded by lateral prominent ridges from the more convex 



