THE BONES IN THE ElfALIOSA.TJEIA. 297 



form, especially in the snout, the lateral position of the exterior 

 nares in Ichthyosaurus, in front of the large circumscribed circular 

 orbits, necessitates the enormous lateral development of the pre- 

 maxillaries and a backward position of the maxillary bones. The 

 lateral relation of the premaxillary bones in Icthyosaurus divides 

 them proxiraally, and allows the nasal bones a large space in which 

 to elongate and widen between them ; while in Porpoises (suppo- 

 sing the bones correctly determined) the nasal bones are small, 

 and only just indent the premaxillary bones behind, and the 

 premaxillary bones, drawn together mesially, allow the maxillary 

 bones to extend external to them along their length, and to carry 

 the teeth. In early life Porpoises, like Ichthyosaurs, carry their 

 teeth in a groove instead of in sockets. 



Wagler and other naturalists have compared the foramen pa- 

 rietale of Ichthyosaurs to the spiracles of Porpoises. And as the 

 foramen descends obliquely forward into the skull, the structures 

 have characters in common. If, then, we suppose the perfora- 

 tions to have become larger with functional activity in the Por- 

 poise and more nearly vertical than in Ichthyosaurus, so as to 

 have advanced forward through the frontal bones, then the me- 

 dian premaxillary bones of the Porpoise would have to be called 

 nasals to bring them into harmony with Ichthyosaurus, while the 

 maxillary bones would become the premaxillaries. But however 

 plausible this interpretation looks in the skull seen from above, 

 it becomes untenable on turning to the palate, where in both 

 types (using the usual nomenclature) the premaxillary bones form 

 the end of the snout, and are parted by the vomers behind, while 

 the maxillary bone in both carries teeth and extends back beyond 

 the alveoli. 



In those Porpoises in which the occipital condyles blend into 

 one long crescent, the single articulation is entirely made by the 

 exoceipital bones, excluding the basioccipital ; while in Ichthyo- 

 saurus the basioccipital forms the entire condyle and excludes 

 the exoceipital bones. 



In the Elephant the external nostrils are far back, but the nasal 

 bones are entirely behind them ; and, after the manner of all mam- 

 mals except the true Whales, the maxillary bones meet mesially 

 on the palate, which they never do in Ichthyosaurs. 



The centrum of a vertebra is not usually quite so long in Ich- 

 thyosaurus as in Porpoises, and diflers in being deeply biconcave 

 (as in fishes), in not having epiphyses, in having the transverse 



22* 



