160 REPORT — 1859. 



In the Pliosaurus the neck- vertebrae are comparatively few in number, 

 short, and flat. The Sauropterygian type seems to have attained its maximum 

 dimensions in this genus, the species of which are peculiar to the Oxfbrdian 

 and Kimmeridgian divisions of the Upper Oolitic system. The Polyptychodon 

 of the cretaceous series also attained a gigantic size. 



M. von Meyer regards the number of cervical vertebras and the length of 

 neck as characters of prime importance in the classification of Reptilia, and 

 founds thereon his order called ' Macrotrachelen,' in which he includes 

 Simosaurus, Pistosaurus, and Noihosaurus with Plesiosaurus. No doubt 

 the number of vertebrae in the same skeleton bears a certain relation to 

 ordinal groups : the Ophidia find a common character therein : yet it is not 

 their essential character; for the snake-like form, dependent on multiplied 

 vertebra?, characterizes equally certain batrachians (Ccecilia) and fishes 

 (Murcena). Certain regions of the vertebral column are the seat of great 

 varieties, in the same natural group of Reptilia. We have long-tailed and 

 short-tailed Lizards ; but do not, therefore, separate those with numerous 

 caudal vertebrae as ' Macroura ' from those with few or none. The extinct 

 Dolichosaurus of the Kentish chalk, with its proccelian vertebrae, cannot be 

 ordinally separated, by reason of its more numerous cervical vertebrae, from 

 other shorter-necked proccelian lizards. As little can we separate the short- 

 necked and big-headed ampliccelian Pliosaur from the ' Macrotrachelians' of 

 von Meyer, with which it has its most intimate and true affinities. 



There is much reason indeed to suspect that some of the Muschelkalk 

 Saurians, which are as closely allied to Nothosaurus as Pliosaurus is to 

 Plesiosaurus, may have presented analogous modifications in the number and 

 proportions of the cervical vertebra?.. It is hardly possible to contemplate the 

 broad and short-snouted skull of the Simosatirus, with its proportionately 

 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 com- 

 paratively short and strong neck. Yet the composition of the skull, its zygo- 

 mata, temporal cavities, and other light-giving anatomical characters, all 

 bespeak the close essential relationship of Placodus to Simosaurus and other 

 so-called • Macrotrachelian' reptiles of the Muschelkalk-beds. I continue, 

 therefore, as in my former 'Report' of 1841, to regard the fin-like modifi- 

 cation of the limbs as a better ordinal character than the number of vertebrae 

 in any particular region of the spine. Yet this limb-character is subordinate 

 to the characters derived from the structure of the skull and of the teeth. 

 If, therefore, the general term Enaliosauria may be sometimes found con- 

 venient in its application to the natatory group of Saurian Reptiles, the 

 essential distinctness of the orders Sauropierygia and Ichthyopterygia, typified 

 by the Ichthyosaurus and Plesiosaurus respectively, should be borne in mind. 



The Plesiosaurus, with its very numerous cervical vertebrae, sometimes 

 thirty in number, may be regarded as the type of the Sauropierygia or 

 pentadactyle sea-lizards. Of all existing Reptiles, the lizards, and amongst 

 these the Old-world Monitors ( Varanus, Fitz.), by reason of the cranial 

 vacuities in front of the orbits, most resemble the Plesiosaur in the structure 

 of the skull, the division of the nostrils, the vacuities in the occipital 



