The general form of the cranium of the Ichthyosaur= resembles that of the ordinary cetaceous dolphin (Delphinia tursio); but the I. tenuirostris rivals the Delphinus gangetieus in the length and slenderness of the jaws. The essential difference in the sea-reptile lies in the restricted size of the cerebral cavity, and the vast depth and breadth of the zygomatic arches, to which the seeming expanse of the cranium is due ; still more in the persistent individuality of the elements of those cranial bones which have been blended into single though compound bones in the sea-mammal. The Ichthyosaur= further differs in the great size of the premaxillary, and small size of the maxillary bones, in the lateral aspects of the nostrils, in the immense size of the orbits, and in the large and nume rous sclerotic plates, which latter structures give to the skull of the Ichthyosaurus its most striking features.
The true affinities of the Ichthyosaur are, however, to be elucidated by a deeper and more detailed comparison of the structure of the skull ; and few collections now afford richer materials for pursuing and illustrating such comparisons than the palaeontological series in the British Museum.* The two supplemental bones of the skull, which have no homologues in existing Crocodilians, are the post-orbital and super squamosal ; both, however, are developed in Arehegosauru.s and the Labyrinthodonts. The post-orbital is the homologue of the inferior division of the post-frontal in those Lacertians —e.g., Iguana, Tejus, Ophisaurus, 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 very 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 0 ripara. 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 walls of the skull of the Ichthyosaurus are the following :—In the posterior region the " foramen magnum," the occipito-parietal vacuities, and the auditory passages ; on the upper surface the parietal foramen and the temporal fosste ; 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 Ichayosawrus.
The orbit is most remarkable in the Ichthyosaurus, amongst reptiles, both 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 lacrymal in front, by the post-orbital 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. The nostril is a longish triangular aperture, with the narrow base behind ; it is bounded by the lacrymal, nasal, maxillary, and premaxillary bones. 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 concavities of the basi-sphenoid, and gradually narrow ing to a point close to the palatine nostrils. These are smaller than in most lizards, and are circumscribed by the palatines, eeto-pterygoid, maxillary, and premaxillary. The pterygo malar fissures are the lower outlets of the temporal fossm ; their sudden posterior breadth, due to the emargination of the pterygoid, relates to the passage of the muscles for attachment to the lower jaw. The parietal foramen is bounded by both parietals and frontals ; its presence is a mark of labyrinthodont and lacertian affinities ; its formation is like that in Iguana and Rhynclincephoirts. The temporal fossfr are hounded above by the parietal internally, by the mastoid and post-frontal externally ; they are of an oval form, with the great end forward. In their relative size and backward position they are more crocodilian than lacertian.