224 PALEONTOLOGY. 



mosal and post-frontal. The super-squamosal may be in like 

 manner regarded as a dismemberment of the squamosal ; were 

 it confluent therewith, the resemblance which the bone would 

 present to the zygomatic and squamosal parts of the mam- 

 malian temporal bone would be very close ; only the squa- 

 mosal part would be removed from the inner wall to the outer 

 wall of the temporal fossa. The super-squamosal, in fact, occu- 

 pies the position of the temporal fascia in Mammalia, and 

 should be regarded as a supplemental sclero-dermal plate, 

 closing the vacuity between the upper and lower elements of 

 the zygomatic arch, peculiar to certain air-breathing Ovvpara. 

 In the Ichthyosaurus it is a broad, thin, flat, irregular-shaped 

 plate, smooth and slightly convex externally, and wedged into 

 the interspaces between the post-frontal, post-orbital, squa- 

 mosal, tympanic, and mastoid. 



The principal vacuities or apertures in the bony walls of 

 the skull of the Ichthyosaurus are the following: — In the 

 posterior region the " foramen magnum," the occipitoparietal 

 vacuities, and the auditory passages; on the upper surface 

 the parietal foramen and the temporal 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 pterygo-malar vacuities. 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 tympanic takes a greater share in the forma- 

 tion of the "meatus auditorius" in many lizards ; in crocodiles 

 it is restricted to that which it takes in Ichthyosaurus. 



The orbit is remarkable for its large proportional size and 

 its posterior position ; in the former character it resembles 

 that in the lizards, in the latter that in the crocodiles. It is 

 formed by the pre- and post-frontals above, by the lachrymal 



