186 THE ANATOMY OF YERTEBRATED ANIMALS. 



of the vertebrse. In the species which have been named 

 Pliosaurus, the vertebrae are wide in proportion to their 

 length, and deeply excavated in front and behind. Fliosaurus 

 attained gigantic dimensions, paddles of some individuals 

 reaching a length of not less than six feet. 



The Triassic genera, JSFothosaiirus^ jSimoscmriis, Pisto- 

 saurus (for a knowledge of the organization of which we are 

 chiefly indejDted to the labors of Hermann von Meyer), appear 

 to have diifered from Plesiosaurus principally in the following 

 respects : 



The connection of the neural arches with the centra of the 

 vertebrge seems to have been looser. The supra-temporal 

 fossae in the skull appear to have been larger in proportion. 

 In these animals, the under-surface of the skull has the same 

 structure as in Plesioscmrus, but a^Dparently lacks the poste- 

 rior fossae ; while there is no doubt whatsoever that the true 

 posterior nares are situated far forward, in the position as- 

 signed to them in Plesiosauriis. 



The pectoral arch of N^othosaiirus^ again, presents a very 

 interesting deviation from the Plesiosaurian type. The cora- 

 coids, indeed, are greatly expanded, and meet by their inner 

 edges, so that the rhomboid al part of the sternum seems to 

 have been wholly absent, and the scapulae have a horizontal 

 prolongation, not quite so long as in Plesioscmrus^ with an up- 

 standing proper scapular part of corresponding shape. But 

 then the ends of these preglenoidal processes are connected 

 together by, and indeed suturally united with, a stout, curved, 

 transverse bar of bone, consisting of three pieces, one small 

 and median, and two very large and lateral, all united firmly 

 together by sutures. There can be little doubt that the con- 

 stituents of this bony bar correspond with the interclavicles 

 and clavicles of Lacertilia and Ichthyosauria. 



III. The Lacerttlta. — Some few Lacertilia^ like the Cha- 

 maeleons and the A-mphisbmna?, are covered by a soft integu- 

 ment ; but, in the majority, there is an epidermic exoskeleton 

 composed of horny plates, tubercles, or spines, or overlapping 

 scales. In some forms (e. g., Scincus, Cydodus) the dermis 

 beneath the horny scales is ossified, and the body has a com- 

 plete armor of bony scutes, corresponding in form with the 

 scales. The dermal ossifications of the head may coalesce 

 with the subjacent bones, but this union of dermal bones with 

 subjacent parts does not occur in other parts of the body. 



The spinal column always contains a considerable number 



