The anterior frontals (2) in the osseous Fishes bound the orbit anteriorly. Between these bones pass out the olfactory nerves, but they are not always distinctly recognisable, being occasionally permanently cartilaginous. In Reptiles these bones are generally distinct, but in Birds and Mammalia they coalesce with the preceding.
The posterior frontals (4) form the posterior margin of the orbit, and are present in Fishes and the Reptilia, but in Birds and Mammifers they are no longer recognisable as distinct bones.
The parietal bones (7) are placed behind the frontals; but these bones do not always touch each other, being separated, especially in Fishes, by the interposition of an azygos bone. from which projects the occipital spine, which is frequently, more especially in Fishes, of very good size : this impair bone, the interparietal of some authors, is in reality the representative of the superior occipitals of Cuvier ( supra occipitals, Owen ;) (8) and in some Fishes, es pecially in the Siluri, where the parietals are totally wanting, their place is supplied by the enormous developement of this element of the skeleton.
The external occipitals (9) contribute to form the lateral portions of the occipital region of the skull, in conjunction with two other pieces called The lateral occipitals (10), which partially bound the foramen magnuin.
The inferior occipital or basilar bone (5) is that which invariably is articulated to the body of the first cervical vertebra, but occasionally in Fishes there are two additional articulations connecting the cranium to the spinal column formed by the lower portions of the lateral occipitals. All these elements of the so-called occipital hone of the human cranium remain permanently distinct in Fishes and Reptiles, and even in the fcetal condition of Birds and 'Mam mals are more or less recognisable; but they soon coalesce into one large piece that enters largely into the formation of the cranial box, and constitute the first or occipital cranial vertebra, as has been already seen.
The sphenoid (6) invariably occupies the cen tral portion of the base of the cranium, and in Fishes and Birds is prolonged anteriorly into a lengthy process which passes beneath the inter orbital septum, which,in these classes of Verte brata, remains most frequently meinbranous.
The alar bones (11; ali-sphenoid, Owen,) represented in the human subject by the greater alw of the sphenoid, are in reality distinct elements of the cranium and are recognizable by several important ch'aracters, especially by their position being joined by suture to the posterior frontral, and, conjointly with the latter bone, to the temporal. Moreover, through these bones the two posterior divisions of the fifth pair of nerves always pass out from the skull. In Fishes and Reptiles they are im portant pieces and quite detached from the sphenoid.
The squamo-temporal bones, Owen (mastoid bones, euv.: 12) in Fishes are manifestly the representatives of the bones so named in Rep tiles. They contribute in conjunction with the posterior frontal, and occasionally with the alar, to furnish the articular surface that sustains the first bone of the palatine and tympanic appa ratus or, in other words, of the masto-tem poral' (23).
The petro-temporal bones, Owen, (13) are in Fishes placed between the mastoid, the lateral, occipital, and the alar bones. They are gene rally of small size, but occasionally, as in the Gadide, very largely developed, descending to reach the inferior occipital and the sphenoid. On the other hand, they are frequently entirely wanting, as for instance, in the Pike, the Carp, and the Eel'.
In all the Reptilia the petro-temporal bones are recognizable as distinct pieces forming part of the cranial box, and become interesting, inasmuch as it is in them that the auditory apparatus is lodged.
In Birds and Marumalia, however, the petrous bones become at an early period inseparably soldered to the other pieces, form ing the so-called " temporal bone. ' The ingrassial bones (14), as they have been named by Genffroy, are, in the human subject, regarded as portions of the sphenoid, although in reality they are distinct elements of the skull. In the higher Vertebrata they are con solidated with the sphenoid, and have received the names of aim minores or apophyses ingrassii. Above these pass out the olfactory and beneath them the optic nerves, a circumstance which in itself sufficiently indicates their real nature. Sometimes, as in the Carp, they are united together inferiorly, so as to form a roof over the optic nerves.