102 FOSSIL REPTILIA OF THE 



resemblance to those parts of the Mammalian ' temporal bone ' would be close, save 

 that the squamous portion would be removed from the inner to the outer wall of the tem- 

 poral fossa. The prosquamosal holds the place of the temporal fascia in Mammals, and 

 should be viewed as a sclerodermal ossification closing, in Ichthyosaurus as in Labyriu- 

 thodon, the vacuity between the upper and lower zygomatic arches, such as exists in Croco- 

 dilia. In Ichthyosaurus the prosquamosal (27') is a broad, thin, flat, irregularly-shaped 

 bony plate, smooth and subconvex outwardly, wedged into an interspace between the 

 postfrontal, postorbital, zygomatic, tympanic, and mastoid bones. 



The chief vacuities in the skull are : — In the occipital region (PI. XXVI, fig. 1), the 



* foramen magnum ' or neural canal of the occipital vertebra ( w ), the ' occipitoparietal,' 

 (op), and the 'auditory' (m) ; on the upper surface (PI. XXIII, fig. 1), the 'foramen 

 parietale ' (/) and the ' temporal fossEe ' (t) ; on the sides (PI. XXIV), the ' orbits ' (o) 

 and the ' nostrils ' (») ; on the lower surface (PL XXV) the ' palato-nares ' ( p »), the 



* interpterygoid ' (s), and the c pterygomalar ' (y) 1 apertures. 



The ' foramen magnum ' is formed by the basi-, ex-, and super-occipitals, the last 

 having an equal share with the exoccipitals ; the basioccipital contributes the least part of 

 any. The occipitoparietal vacuities are larger than in Crocodilia, smaller than in Lacer- 

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

 and mastoids, below by the paroccipitals. 



The auditory aperture, or ' meatus ' (PL XXVI, 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 approach the Ichthyosaurs, in the latter the Crocodiles. 

 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 (Stellio, Agama) 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 Ichthyo- 

 saurus the nostril (n) 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. 



1 ' Pterygomaxillary ' in Crocodiles and Lizards, ' Anat. of Vertebrates,' vol. i, pp. 156 and 157, 

 fig. 98, y. 



