60 BRITISH FOSSIL REPTILES. 



part of any. The occipito-parietal vacuities are larger than in Crocodilia, smaller than in 

 Lacertilia ; they are bounded mesially by the ex- and superoccipitals, laterally by the 

 parietals and mastoids, below by the paroccipitals. 



The auditory aperture, or ' meatus ' (Tab. XXII, fig. 1, m), is bounded by the tympanic 

 and zygomatic. The tympanic takes a greater share in the formation of the ' meatus 

 auditorius ' in many Lizards ; in Crocodiles it is restricted to that which it takes in 

 Ichthyosaurs. 



The orbit is remarkable for its large relative size and backward position : in the 

 former character the Lizards, in the latter the Crocodiles, approach the Ichthyosaurs. 

 The cavity is formed by the pre- and postfrontals above, by the lacrymal in front, by 

 the postorbital behind, and by the long and slender malar below. In Crocodiles, and 

 in most Lizards, the frontal or mid-frontal enters into the formation of the orbit, and in 

 some Lizards (Sieliio, Jj/avia) the maxillary also. In Chameleons, both the frontal and 

 the maxillary are excluded from the orbit. 



The external nostrils are not homologous with the single medial one in the 

 Crocodiles, but answer to the parial nostrils in Lizards, and to the supplementary aper- 

 tures bounded by the nasal, lacrymal, and maxillary bones in the Teleosaurs. In 

 Lizards the lacrymal is usually excluded by the maxillary from the nostril. In Iclithyo- 

 saimis the nostril («) is a longish triangular aperture, with the narrow curved base behind ; 

 it is bounded by the lacrymal, nasal and premaxillary (22), sometimes also by the 

 maxillary bones, and is usually distant from the orbit by less than its own long diameter. 

 Like the orbit, the plane of its outlet is almost vertical. 



The interpterygoid vacuity (Tab. XXI, fig. 1, s) is a very long and narrow triangular 

 one; its base is behind, and is bounded by the anterior concavities of the basisphenoid. 

 As it advances it is divided for a certain extent by the presphenoidal rostrum ; the lateral 

 boundaries are due exclusively to the pterygoids, which, converging, reduce the aperture 

 to a point. In this formation, Ichthyosaurus resembles Iguana, but in Varantis the 

 palatines enter into the formation of the palatal vacuity in question. 



The pterygomalar vacuity (Tab. XXI, fig. 1, y) may be described as the lower outlet 

 of the temporal fossa ; it is bounded laterally by the zygomatic and malar, chiefly the latter 

 bone; posteriorly by the tympanic, anteriorly by the maxillary, mesially by the ectopterygoid 

 and pterygoid ; the outer deep emargination of the latter bone relates to the passage of the 

 large temporal muscles for insertion into the mandible. The larger proportion which the 

 maxillary contributes to this vacuity in recent Saurians suggested the term ' pterygo- 

 maxillary,' applied to it in the undercited work.^ 



The palato-nares (ib., pn) are relatively smaller than in most Lizards, and are circum- 

 scribed by the palatine, ectopterygoid, maxillary, and premaxillary bones. In Lizards the 

 ectopterygoid is excluded from its formation, and the position of the palato-naris is more 



1 'Anatomy of Vertebrates,' vol. i, p. 156. 



