394 Miscellaneous. 



occupies the position of the temporal fascia in Mammalia, and should 

 be regarded as a supplemental scleroderma! plate, closing the vacuity 

 between the upper and lower elements of the zygomatic arch, pecu- 

 liar to certain air-breathing Ovipara. It is a broad, thin, flat, irre- 

 gular-shaped plate, smooth and slightly convex externally, and wedged 

 into the interspaces between the postfrontal, postorbital, squamosal, 

 tympanic and mastoid. 



The principal vacuities or apertures in the bony walls of the skull 

 of i\iQ Ichthyosaurus are the following : — In the posterior region, the 

 "foramen magnum," the occipito-parietal vacuities, and the auditory 

 passages ; on the upper surface are the parietal foramen and the tem- 

 poral fossae ; on the lateral surfaces, the orbits and nostrils, the 

 plane of the aperture in both being vertical ; on the inferior surface, 

 the palato-nasal, the pterygo-sphenoid, and the ptery go-malar 

 vacuities. 



The "foramen magnum" is formed by the basi-, ex- and super- 

 occipitals, the last having as much, or rather more share, than the 

 ex-occipitals ; the basi-occipital contributes a very small part below. 

 The occipito-parietal vacuities are larger than in Crocodilia, smaller 

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

 super-occipitals, externally by the parietal and mastoid. The auditory 

 apertures are bounded by the tympanic and squamosal. The tym- 

 panic takes a greater share in the formation of the " meatus audi- 

 torius" in many Lizards ; in Crocodiles it is restricted to that which it 

 takes in Ichthyosaurus. 



The orbit is most remarkable in the Ichthyosaurus amongst rep- 

 tiles, both for its large proportional size and its posterior position ; 

 in the former character it resembles that in the Lizard, in the latter 

 that in the Crocodile. It is formed by the pre- and post-frontals 

 above, by the lacrymal in front, by the postorbital behind, and 

 by the peculiar long and slender malar bar below. In crocodiles and 

 in most lizards, the frontal enters into the formation of the orbits, 

 and in lizards the maxillary also. In chameleons the frontal is ex- 

 cluded above, and the maxillary below, from the orbit, as in Ich- 

 thyosaurus. 



The nostril is a longish triangular aperture, with the narrow base 

 behind : it is bounded by the lacrymal, nasal, maxillary and pre- 

 maxillary, it is proportionally larger than in the Plesiosaurus, and is 

 distant from the orbit about half its own long diameter. Like the 

 orbit, the plane of its outlet is vertical. 



The pterygo-palatine vacuities are very long and narrow, broadest 

 behind, where they are bounded, as in lizards, by the anterior con- 

 cavities of the basi-sphenoid, and gradually narrowing to a point, 

 close to the palatine nostrils. 



These are smaller than in most lizards, and are circumscribed by 

 the palatine, ectopterygoid, maxillary, and premaxillary. The 

 pterygo-malar fissures are the lower outlets of the temporal fossae : 

 their sudden posterior breadth, due to the emargination of the ptery- 

 goid, relates to the passage of the muscles for attachment to the lower 

 jaw. 



