Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/468

Rh 450 REPTILES [ANATOMY. very continuous open case, with no mobility save that of the lower jaw; or it may consist of bones several of which are so conditioned as to allow much mobility to several other parts. In all cases, however, the side wall of the anterior part of tho cranial cavity is very imperfectly ossified. inCroco- The skull of the Crocodilia is distinguished from that lilians ; o f a n other Reptiles by its very extensive bony palate, which exceeds in completeness that of even any Mammal save the Anteaters and Cetaceans, for not only the maxillse and palatines but also the pterygoids all as expanded bony plates suturally united concur in its com- position in all existing Crocodilians (though not in the extinct Teleosaunis or Belodon], and the pterygoids sur- round the posterior nares. The skull forms a solid whole composed of bones united by suture except at the antero- lateral and median boundaries of the cranial cavity, where it is cartilaginous or membranous. There is an interorbital septum. The quadrate bone is immovably fixed and of large size, and unites with the pterygoid, but by its upper and inner surface only. The alisphenoid is a large broad bone. The tympanic cavity is completely enclosed by the prootic and opisthotic (the latter being united with the exoccipital), the squamosal, the postfrontal, and the basioc- cipital and basisphenoid. It opens into the mouth by three apertures, one median and two lateral, 1 which terminate complex canals having communications among each other. There are two lateral, quasi-zygomatic arches to the skull an upper one formed by the postfrontal and squamosal, and a lower one by the maxilla, jugal, quadra to-jugal, and quadrate. There is no foramen in the parietal region of the skull, and there is an azygous parietal bone and frontal. On either side of the hinder region of the skull the periotic and exoccipital bones form large parotic processes. There is a distinct perforated lachrymal. There are two vomers, which are generally hidden in the palate by the junction of the extensive maxillae and palatines. There are a pair of nasal bones. Various cranial bones are pneumatic, including the os articulare of the mandible. The hyoid is very simple, and consists only of a broad cartilaginous or partly osseous basihyal, with two bony cornua, not directly connected with the skull. There is a very small carti- laginous stylohyal on the upper hinder part of the quad- rate. a Hat- Hatteria. The skull of this living type of an extinct ena > order resembles that of the Crocodilia in that there is a lower zygomatic arcade formed by the quadrato-jugal bone interposed between the malar and the quadrate, as well as a superior zygomatic arcade formed by the squamosal and postfrontal, and in that the quadrate bone is immov- ably fixed between the pterygoid, squamosal, and quadrato- jugal. The palate is pretty complete with wide plate-like ossifications, still it is much less so than in the Crocodiles ; but the posterior nares are much more anteriorly situated very near the anterior end of the palate and are on each side, being each bounded by the premaxilla in front, the vomer internally, the maxilla externally, and the palatine behind. At the side of the skull we find a bone distin- guished as the "columella," which passes upwards from above the suture between the pterygoid and quadrate to the parietal, to which it is attached by a slip of cartilage. It is a flattened bone, somewhat expanded above and below, and constricted towards its middle. The lateral wall of the skull at the part which in the Crocodile is occupied by the alisphenoid, and in front of that part is not osseous but fibro-cartilaginous. There is an interorbital septum. The postero-lateral region of the skull consists, as in the 1 For a full description with good figures of this very complex structure, see Owen, Phil. Trans., February 28, 1850, vol. cxl. p. 521, pis. 40-42. Crocodiles, of two outstanding " parotic processes," made up of the exoccipital, prootic, and opisthotic bones, beneath which is the "columella auris." The basi- sphenoid sends down two processes to abut against the pterygoids. The parietal is perforated by a small median ft FIG. 10. Skull of Hatteria (after Gunther). 1, ventral aspect; 2, lateral aspect; fontanelle. The premaxillae are separate, and together form a sort of beak, their large teeth becoming thoroughly anchylosed and united with the bones supporting them. The nasals are double, and each sends forth a process (some- what as in Birds) from its outer anterior angle. The Lacertilian skull is formed mainly upon one of two in I diverging types of structure (1) that of ordinary Lizards, tilit and (2) that of Chamseleons. In both the quadrate bone is almost always movable and the inferior zygomatic arcade is wanting, though generally represented by a ligament ; the palate is incompletely ossified, and the rather anteriorly situated posterior nares bounded internally by the bifold vomers. In the ordinary Lizard type the skull has the appearance of consisting of a system of osseous bars con- necting the solid occipital parts (with its pair of parotic processes) with a flattened cranial roof and the more or less well ossified snout. The skull has an interorbital septum. The lateral walls of the cranium are, as in Hatteria, fibro-cartilaginous, though they may contain some insig- nificant ossifications ; and a " columella," as in Hatteria, generally ascends from the pterygoid to the parietal. The last-named bone sends a backward prolongation to the parotic process and squamosal, and is movably united to the occipital ; and thus, through the imperfect ossifica- tion of the cranial parietes, the facial part of the skull is capable of more or less flexion upon the occipital part