. Elements of Comparative Anatomy. SKULL OF VERTEBEATA. 451 Otliers are formed in the mucous membrane of tlie buccal cavity, that is, in tbose parts of the cartilaginous arches of the mandibular and branchial skeleton which aid in forming the boundaries of this cavity. The origin of these bones may be seen, in the Amphibia, to be due to the concrescence of tooth-like structures, which are, indeed, similar to the dermal denticles, and which are also found on the covering of the buccal and branchial cavities in the Sela- chii. Dentigerous plates are formed by the fusion of some of these den- tic
. Elements of Comparative Anatomy. SKULL OF VERTEBEATA. 451 Otliers are formed in the mucous membrane of tlie buccal cavity, that is, in tbose parts of the cartilaginous arches of the mandibular and branchial skeleton which aid in forming the boundaries of this cavity. The origin of these bones may be seen, in the Amphibia, to be due to the concrescence of tooth-like structures, which are, indeed, similar to the dermal denticles, and which are also found on the covering of the buccal and branchial cavities in the Sela- chii. Dentigerous plates are formed by the fusion of some of these den- ticles, and these afford points of support for the necessary primordial skeletal deposits, and gradually become con- nected with them. In some of these plates the denticles are retained; in others they disap- pear, or are no longer developed, so that their product — that is, the bone — is alone de- veloped; and this, by investing or growing around the cartilage, forms a perichondrial ossification. The origin, therefore, of a large part of the osseous ce- phalic skeleton can be. Fig. 242. Skull of Salmo salar. A Side view. B Vertical median section. The cartilaginous parts are hatched, while the bones derived from the primordial cranium are dotted. Oh Basi-occipital. 01 Exoccipital. Os Supra-occipital. Sq Squa- mosal. Eji 0 Epiotic. Fr 0 Petrosal. Sh Basi- sphenoid. Ah Alisphenoid. Or S Orbito-sphenoid. Fa Prefrontal. Fp Postf rental. Fr Frontal. Na Nasal. Ps Parasphenoid. Vo Vomer. Px Pve- masilla. gl Articular surface for the Hyoman- dibular. Eth Ethmoidal cartilage, vag Foramen for the exit of the vagus. ,.^^..w .^-w. can be explained by the relations, which the ossifications of the external integument, and of the investment of the buccal cavity, have to it. The phylogeny, however, of a few of the cephalic bones is as yet unknown. Hertwig, 0., Ueber das Zahusystem der Amphibien, Bd. XI. Supplement. § 343. Arch. f. mikr. Anat. With regard to the various bones, we divide^ the
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