Reverting to the lower surface of the conduit), behind the maxillaries and the frontals, posterior to the two sides of the comer, are the palatines, surrounded behind and externally by the pterygoid bones, which last extend along the external border of the palatine to the maxillary bones. The rest of the pterygoids covers the lower surface of the cranium between the two tympanic cavities and the two temporal alw, leaving exposed to view behind only a triangular part of the body of the sphenoid. The olfactory and optic nerves have their exit by the cartilaginous septa of the cranium, and not by any particular opening in the skull. Cuvier thinks that it is the same with the third and fourth pairs: the sixth goes forth by a small canal of the body of the sphenoid bone. The fifth pair has a great hole between the pctrous bone and the temporal ala divided into two externally. There is at the external border of the palatine bone a hole analogous to the pterygo-palatine.
Internally, the cerebral cavity is higher than it is wide; the bottom of it is very entire : but, in front, in the sphenoid, there is a deep fosset for the pituitary gland, a kind of saddle. From the sides of this part spring the cartilaginous septa, which in going to form a junction with the ante-cerebral partition of the frontal bone, close the cavity of the cranium, support the whole anterior part of the encephalon, and occupy the place of the cribriform plate, of the orbital alit; or otherwise, the anterior sphenoid, and the greater part of the temporal ake, of which another considerable part is replaced by the descending portions of the parietal, so that what remains does not participate in the formation of the chamber of the cranium except a little in front of the hole for the fifth pair of nerves. There is no more bony trace of the anterior sphenoid than in the crocodile.
In the Emydes, or ordinary Fresh-Water Tortoises, the head is more flattened. The principal frontals, although they are wider than they are long, do not always reach to the border of the orbit, as is, for example, the case in the Testudo (Ciatudo) Europa; the posterior frontal is wider. The frame of the tympanum is not complete, and in lieu of a hole there is a fissure for the passage of the ossiculum audittts from one hollow of the cavity to the other. The basilary and palatine regions form but one plane ; the palatines not being even concave. Cuvier observes that Test udines script a, pieta, scabra, dorsata, centrata, clause, and vergulata, belong to this category. Certain Emydes, he remarks, Emys expanse for instance, tend to the Sea Tortoises, or Turtles, and the Fresh-Water Tortoises, and yet exhibit characters peculiar to themselves.
In the Trionyces, or Soft Tortoises, the skull is depressed, and elongated backwards ; the muzzle, pointed in certain species (that of the Nile for instance), is short and rounded in some others. The intermaxillary bones are very small, and have neither nasal nor palatine apophysis ; there is behind them a large incisive hole. The maxillaries unite upon the palate for a rather long space, so that the posterior nostrils are more backwards than in the Land-Tortoises. The palatines do not unite below to prolong the palate ; they are hollowed into a demi-canal anteriorly, and less extended than in the Land-Tortoises.
The principal character of the Marine Tortoises, or Turtles, is that a lamina of their parietal, their posterior frontal, their mastoidean, their temporal, and their jugal, unite together, and with the tympanic cavity, by sutures, to cover the whole region of the temple with a bony roof, which has no solution of continuity. Their muzzle being shorter than in other tortoises, and their orbits much longer, their nasal cavity is smaller, and as wide as it is high and long. Its posterior wall belongs entirely to the anterior frontals,' and it is between them that the olfactory nerves are introduced. The bony tubes of the back nostrils commence in the lower part of this posterior partition, and, like the palatines, have a palatine part or lower lamina; these tubes are rather longer, more directed backwards, and bear less resemblance to simple holes. It results also from the size of the orbit that the inter-orbital membranous or cartilaginous space is more extended.
The most heteroclite skull among the tortoises is that of the Mats mate (Testudo fimbriata, Chelys firabriate). Extraordinarily large and flat, it seems to have been crushed. The very small orbits are close to the end of the muzzle. The posterior region of the cranium is elevated ; and the two tympanic bones, in form of trumpets, widen out on each side of the cranium. The temple is a wide horizontal fosse, not deep, and not at all covered, except behind by the union of the posterior angle of the parietal with the mastoidean bone ; and, what is peculiar, Cuvier observes, to this sub-genus, this fossa is not framed in externally, because there is no temporal bone, or at least it is reduced to a simple vestige. The two maxillaries form together a transversal arch, in the middle of which, below, is a single inter maxillary, and, above, the external aperture of the nostrils, which is continued into a small fleshy proboscis. The two palatine bones, and, between them, the vomer, fill below the concavity of this arch, and have in front the two back nostrils well separated, but which the palatines do not encircle below. At the posterior border of the palatine is a rather large pterygo-palatine hole. The anterior and posterior frootals form the upper part of the orbits. The principal frontals advance between the anterior frontals to the edge of the external nostrils. There is no more nasal bone than in the other tortoises. The jugal proceeds from the posterior angle of the orbit between the maxillary and posterior frontal, beyond which it does not go, touching a little behind and below the pterygoidcan; but not forming any projection behind to border the temple. This last is in this manner separated from the orbit by a postorbital branch of excessive width, and which takes in the totality of the posterior frontal and the jugal bones. The posterior frontal articulates itself to the pterygoidean by its external posterior angle. The rest of its posterior border is free, and is continued with that of the parietal to cover a wide and flat canal of communication, proceeding from the temple to the orbit, and formed below by the pterygoidean and palatine bones. The two pterygoideans arc enormous. They form the greatest part of the base of the cranium and of the bottom of the temple. Their external border is curved in its anterior part for I its continuation with the free border of the posterior frontal ; there are neither orbital nor temporal age. The parietal bones, which form above a great rectangle, unite by their descending portions to the palatines, the pterygoideans, the petrous, and the upper occipital bones. They form by themselves nearly the whole roof of the cranium. Following the pterygoidean, the temple is bounded behind by the tympanic bone or the tympanic cavity, which resembles in part a trumpet. The frame of the tympanum is complete. A hole in the posterior wall suffers the ossieuhun to pass into the second chamber, which, in the skull, is only a long groove of the posterior surface of the cavity, which terminates in a hollow, in the formation of which the petrous bone, the external occipital, and the lateral occipital concur. It is not closed behind, except by cartilage and membranes; and in the wall of the side of the cranium are pierced the two fenestrie, as ordinarily. Above this hole of the first chamber, by which the ossiculinn passes, is another which conducts into the maetoidenn cellule, which, on account of the outward projection of the tympanum, is found within and not behind. The occipital spine is a short vertebral crest, and the mastoidean tubercles are transversal crests, which belong entirely to the inestoidean. Even iu largo individuals the six occipitals ordinary to the tortoises may be distin guished. Below, the smooth and nearly plane cranium presents a sort of regular compartment, formed of the interrnaxillaries, the maxillaries, the Fouler, the palatines, tho pterygoidenus, the sphenoid. the petrous bones, the tympanic cavities, the basilary, and the lateral and external occipitals. Behind the ceiling of the temple the petrous bone forms a square compartment between the pterygoidean, the tympanic cavity, the external occipital, the superior occipital, and the parietal bones.