In the Elasmobranchii (sharks and rays) and Holocephali (Chimaera) among the fishes the skull is still a complete cartila ginous box, though calcification of the cartilage often takes place. Taking the skull of the dogfish as a type, two large olfactory cap sules are seen in front, and behind these the cranial brain-box is narrowed, being excavated at its sides for the great orbits. More posteriorly the auditory capsules widen the skull, and on the pos terior (caudal) aspect the foramen magnum is seen with an occi pital condyle on each side of it for the first vertebra to articulate with. On the upper (dorsal) surface of the skull are two apertures in the middle line; the more anterior of these forms a rudimentary median orbit for the pineal eye (see BRAIN). The posterior f on tanelle is a depression which leads into two lateral tubes, each of which passes into the auditory capsule and is known as an aque ductus vestibuli (see EAR).
In the cartilaginous ganoid fishes (sturgeon), which, like the elasmobranchs, are of great antiquity, the chondro-cranium is partly ossified so that ali- and orbito-sphenoids are found ; in addition to this a large number of dermal bones have made their appearance, such as nasals, frontals, parietals, supra and post tem porals, while in the roof of the mouth and pharynx a long mem brane bone, the parasphenoid, is formed, and lies ventral to and strengthens the cartilaginous base of the skull. These fish are important morphological landmarks, because in them the almost unchanged chondro-cranium coexists with a dermal ectocranium.
In the bony ganoids such as the "bow fin" (Amia) the dermal bones are still more numerous and, among others, squamosals, pro-otics and exoccipitals appear. These fish are also remarkable for a fusion of the anterior part of the vertebral column with the occipital region of the skull, an arrangement recalling that in the skull of the calf embryo mentioned in the section on em bryology.
In the bony fishes (Teleostei) the membrane or dermal bones are still more numerous, and many of them are unrepresented in the mammalian skull, while others, which are there quite rudi mentary, are very large. The chondro-cranium tends to disappear in the vault, but the base is fully ossified. Among other cartilage bones the five ossifications of the auditory capsule are seen, the pro-, epi-, opisth-, pter- and sphen-otics, all of which are found as centres of ossification in man. In the cod, for example, the sphenotic, which is represented in man by the little lingula sphen oidalis, is larger than the alisphenoid.
In the Dipnoi (mud-fish) the chondro-cranium is very slightly ossified, only exoccipitals being found, but there is the same coalescence with anterior vertebrae which was noticed in the ganoids. Dermal bones are plentiful.
In the Amphibia the chondro-cranium persists and is only ossified in front by the girdle bone or sphenethmoid, and behind by the pro-otics and exoccipitals, the latter of which bear the two condyles. The anterior fontanelle is well marked in the chondro-cranium, but is completely overlaid and concealed by the dermal fronto-parietals. The membrane bones though large are much less numerous than in the bony fishes.
In the Reptilia the skull varies immensely in the different orders, but speaking broadly, the chondro-cranium is less distinct than in the Amphibia, except in the ethmoidal region. In the base of the skull the basioccipital and basisphenoid are tending to replace the membranous parasphenoid, and instead of two exoccipital condyles only one in the mid line is present, though this in many forms (e.g.,
Chelonia) consists of three parts, a median borne on the basioc cipital and two lateral on the exoccipitals. The parietal foramen is usually definitely marked in the dermal part of the skull and forms a median orbit for the pineal eye; this is especially the case in the Lacertilia (lizards). Except in the Ophidia (snakes) and Amphisbaenidae (worm-like lizards) there is a fibro-cartilag inous septum between the orbits so that the cranial cavity does not reach forward to the ethmoidal region. The pro-, epi- and opisth otic bones are all developed, but the epiotic usually fuses with the supra-occipital and the opisthotic with the exoccipital.
In the Crocodilia the first attempt at pneumaticity is seen in the basisphenoid, which is traversed by a complicated system of Eustachian passages leading eventually to the tympanum. In the class Ayes the general scheme of the reptilian skull is maintained, though the bones fuse together very early, thus obliterating the sutures between them. Almost all of them have air in their interior, and so are said to be pneumatic.
The single occipital condyle, if looked at in a young specimen, is seen to consist of a basioccipital and two exoccipital elements, though these are indistinguishable in the adult. The parasphenoid is represented by a broad plate which is called the basitemporal. The pro-, epi- and opisth-otic bones fuse together to form the auditory capsule.
In the Mammalia the calvaria varies considerably in the differ ent orders, the characteristic features being best marked in adult males. Usually the different bones are interlocked by sutures, as in man, until adult life, but in some orders (e.g., Monotremata, Edentata and Carnivora) they fuse together quite early.
In the basicranium the cartilage bones presphenoid, basisphen oid and basioccipital, are so well developed that the parasphenoid has disappeared. In the basisphenoid of the rabbit the cranio pharyngeal canal (see section on embryology) persists as a fora men at the bottom of the pituitary fossa. In the lower orders the face lies well in front of the brain case, as it does in reptiles and amphibians, but as the Primates are reached the increasing size of the calvaria causes it to overlie the face. Many of the bones are pneumatic, the process reaching its maximum in the elephant and the adult male gorilla. The periotic capsule blends with the squamosal and tympanic to form the petrous bone, though it is practically only in man that the second visceral arch ossifies on to this as a styloid process. There are usually two occipital condyles which have basi- and exoccipital elements, though there are many mammals in which there is one large crescentic condyle surround ing the anterior half of the foramen magnum.
Ossification of the processes of the dura mater occurs in the tentorium cerebelli of the carnivora and in the falx cerebri of the ornithorhynchus and porpoise. The orbits are in most mammals continuous with the temporal fossae. Sometimes, as in many of the ungulates and in the lemurs, they are outlined by a bony ring, but it is not until the higher Primates are reached that the two cavities are shut off and even then a vestige of their original continuity remains in the spheno-maxillary fissure.