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 prosquaniosal holds the place of the temporal fascia in Mammals, and 
should be viewed as a sclerodermal ossification closing, in Ichthyosaurus as in Lahyrin- 
thodon, the vacuity between the upper and lower zygomatic arches, such as exists in Croco- 
dilia. In Ichthyosaurus the prosquaniosal (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 (^), the ‘ occipito-parietal,’ 
(o jj), and the ‘auditory’ (w?) ; on the upper surface (PI. XXIII, fig. 1), the ‘foramen 
parietale ’ (/) and the ‘ temporal fossae ’ (r) ; on the sides (PI. XXIV), the ‘ orbits ’ (o) 
and the ‘ nostrils ’ («) ; on the lower surface (PI. XXV) the ‘ palato-nares ’ 
‘ interpterygoid ’ (s), and the ‘ pterygomalar ’ (y)^ 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 occipito-parietal vacuities are larger than in Grocodilia, 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 ’ (PI. XXVI, fig. 1, »«), 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 {iStetlio, Ayama) 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 (re) 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. 
^ ‘ Pterygoma.\illary ’ ia Crocodiles and Lizards, ‘ Anat. of Vertebrates,’ vol. i, pp. 156 and 157, 
fig. 98, y. 
