204 PALEONTOLOGY 



in existing Crocodilians, are the post-orbital and super- 

 squainosal ; both, however, are developed in Archcgosaurus 

 and the Labyrinthodonts. The post-orbital is the homologue 

 of the inferior division of the post-frontal in those Lacertians 

 — e.g., Iguana, Tejus, O'phisaurus, Anguis, in which that bone 

 is said to be divided. But in Ichthyosaurus the post-orbital 

 resembles most a dismemberment of the malar. Its thin 

 obtuse scale-like lower end overlaps and joins by a squamous 

 suture the hind end of the malar : the post-orbital expands as 

 it ascends to the middle of the back of the orbit, then gradually 

 contracts to a point as it curves upward and forward, articu- 

 lating with the super-squamosal and post-frontal. The super- 

 squamosal may be in like manner regarded as a dismember- 

 ment of the squamosal ; were it confluent therewith, the 

 resemblance which the bone would present to the zygomatic 

 and squamosal parts of the mammalian temporal would be 

 veiy close ; only the squamosal part would be removed from 

 the inner wall to the outer wall of the temporal fossa. The 

 super-squamosal, in fact, occupies 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 Ovipara. In the Ichthyosaurus it is a broad, thin, 

 flat, irregular-shaped plate, smooth and slightly convex exter- 

 nally, and wedged into the interspaces between the post-frontal, 

 post-orbital, squamosal, tympanic, and mastoid. 



The principal vacuities or apertures in the bony Mails 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 



