ANATOMY OF VERTEBRATES. 159 



in most Lizards, and are circumscriljed by tlie palatines, ecto- 

 pterygoid, maxillary, and premaxillary. The pterygomalar fis- 

 sures are the lower outlets of the temporal fossse ; their sudden 

 posterior breadth, due to the emargination of the pterygoid, relates 

 to the passage of the muscles for attachment to the lower jaw. 

 The parietal foramen is bounded by Ijoth parietals and frontals, 

 11 ; its presence is a mark of labyrinthodont and lacertian affini- 

 ties ; its formation is like that in Ifjuana and Rhynchoeepluilus. 

 The occipitoparietal vacuities are larger than in Crocodilia, smaller 

 than in LdcertiUa ; they are bounded internally by the basi-, ex-, 

 and super-ocoipitals, externally by the parietal and mastoid. The 

 auditory apertures are boimded by the tympanic and squa- 

 mosal : the tympanic, 28, takes a greater share in the formation 

 of the ' meatus auditorius' in Lizards ; in Crocodiles the bone 28 

 is restricted to that which it takes in Ichtlujosaurus.^ 



In comparing the jaws of the Ichthyosaurus tenuirostris with 

 those of the gangetic Gharrial, an equal degree of strength and 

 of alveolar border for teeth result from two very different propor- 

 tions in which the maxillary and premaxillary bones are comljined 

 together to form the upper jaw. The prolongation of the snout 

 is the same : the dilference of structure relates to the collective 

 tendency <if the affinities of the lehthyosaurus to an antecedent 

 ha;mat(5cryal type of structure still partly shown by Lizards. 

 The l)ackward or antorbital position of the nostrils, lilce that in 

 Avhales, is related to the marine existence of the Ichthyosaurs. 

 But in the Labyrinthodonts, in which the nostrils are nearer the 

 fore part of the head, their anterior boundaries are formed by 

 the premaxillaries, as in modern Lizards : it appears, therefore, 

 to be in conformity with these affinities that the premaxillaries 

 of the Ichthyosaur should enter into the same relation with the 

 nostrils, although this involves an extent of anterior develope- 

 ment proportionate to the length of the jaws, the forward pro- 

 duction of which sharp-toothed instruments fitted the Ichthyosaur, 

 like the mcjdern Dolphin for the prehension of agile fishes. 



§ 36. Shull of Dicynodontia. — The skull of the Dicynodon, 

 fig. 99, is articulated with the atlas by a single condyle, formed by 

 tlie liasi- and ex-occipitals in equal proportions : the latter have 

 coalesced, as in the Crocodiles, with the paroccipitals. The parie- 

 tals form one bone, perforated by a small ' foramen parietale ' close 

 to the coronal suture. The frontals, ii, contribute a share to the 

 superorbital border ; their median suture is distinct, as is that 



' CXLVII. p. 388. 



