80 SCIENTIFIC AND LITERARY NOTES. 
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 analo- 
gous modifications in the number and proportions of the cervical vertebra. It is 
hardly possible to contemplate the broad and short-snouted skull of the Simosau- 
rus, with its proportionably 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 teeeh, of a shape 
indicative of their adaptation to crushing molluseous or ecrustaceous 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 sup- 
position 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 bespeak the close essential relationship of 
Placodus to Simosaurus and other so-called Macrotrachelian reptiles of the Musch- 
elkalk beds. Prof. Owen continued, therefore, as in his Report of 1841, to regard 
he fin-like modification of the limbs as a better ordinal character than the number, 
of vertebrze in any particular regoin 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 Sauropterygii and Ichthyopterygii, typified by the 
Ichthyosaurus and Plesiosaurus respectively, should be borne in mind. The 
Plesiosaurus, with its very numerous cervical vertebree, sometimes thirty in number 
may be regarded as the type of the Sauropterygii, 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; as in the division of the 
nostrils, the vacuities in the occipital region between the exoccipitals and tympanics, 
the parietal foramen, the zygomatic extension of the post-frontal, the palato- 
maxillary, and pterygo-sphenoid vacuities in the bony palate; and all these are 
lacertian characters as contradistinguished from crocodilian ones. But the antor- 
bital vacuities, between the nasal, prefrontal, and maxillary bones, are the sole 
external nostrils in the Plesiosaurs. The zygomatic arch abuts against the fore 
part of the tympanic and fixes it: a much greater extent of the roof of the mouth 
is ossified than in lizards, and the palato-maxillary and pterygo-sphenoid fissures 
are reduced to small size. The teeth, finally, are implanted in distinct sockets. 
That the Plesiosaur had the “head of a lizard” is an emphatic mode of expressing 
the amount-of resemblance in their cranial conformation. The crocodilian affinities, 
however, are not confined to the teeth, but are exemplified in some particulars of 
the structure of the skull itself. In the simple mode of articulation of the ribs 
the lacertian affinity is again strongly manifested ; but to this vertebral character 
such affinity is limited. All the others exemplify the ordinal distinction of the 
Plesiosaurs from known existing reptiles. The shape of the joints of the centra ; 
the number of vertebrae between the head and tail, especially of those of the neck ; 
