158 ANATOMY OF VERTEBRATES. 



advance of the interpterygoid ; and more rarely there occurs an 

 ' intervomerine' vacuity still more in advance. 



Thus there are definable and nameable, in the bony palate of 

 reptiles, the ' pterygo-maxillary,' ' palatonarial,' ' premaxillary,' 

 'interpterygoid,' 'interpalatal,' and 'intervomerine' vacuities or 

 foramina — more or less valuable as characters of recent and 

 extinct species. 



§ 35. Skull of Ichthyopterygia. — Amongst the illustrations of 

 extreme varieties in the reptilian skull which Paleontology has 

 brought to light, may be cited the Ichthyosaurus, the Dicynodon, 

 and the Pterodactylus. 



That of the first combines in a peculiar manner some piscine 

 with reptilian characters. It differs from all existing Heptilia in 

 the great size of the premaxillary, fig. 105, 22, and small size of 

 the maxillary, 21 ; in the lateral aspects and antorbital position of 

 the nostrils ; in the immense size of the orbits, and in the large and 

 numerous sclerotic plates, which latter structures give to the skull 

 of the Ichthyosaurus its most striking features. 



The two supplemental bones of the skull, which have no homo- 

 logues in existing Crocodilians, are the postorbital and super- 

 squamosal ; both, however, are developed in Archegosaurus and 

 the Labyrinthodonts. The postorbital is the homologue of the 

 inferior division of the postfrontal in those Lacertians — e. g., 

 Iguana, Tejus, Ophlsaurus, Anguis, in which that bone is said to 

 be divided ; but in Ichthyosaurus it more resembles a dismember- 

 ment of the malar, 26. Its thin obtuse scale-like lower end over- 

 laps and joins by a squamous suture the hind end of the malar : 

 the postorbital 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, articulating with the supersquamosal and post- 

 frontal, 12. The supersquamosal may be in like manner regarded 

 as a dismemberment of the squamosal, 27 ; were it confluent there- 

 with, the resemblance which the bone would present to the zygo- 

 matic and squamosal parts of the mammalian temporal bone would 

 be very close ; save that the squamous part would be removed 

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

 is bounded by the lacrymal, 73, nasal, 15, maxillary, 21, and pre- 

 maxillary, 22, bones. It is distant from the orbit about half its own 

 long diameter. Like the orbit, the plane of its outlet is vertical. 

 The pterygo-maxillary vacuities are very long and narrow, 

 broadest behind, where they are bounded, as in Lizards, by the 

 anterior concavities of the basisphenoid, and gradually narrowing 

 to a point close to the palatine nostrils. These are smaller than 



