SKULL OF VERTEBRATA. 



451 



Others are formed in the mucous membrane of the buccal cavity, 

 that is, in those 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 



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. Ob Basi-occipital. 

 01 Exoccipital. Os Supra-occipital. Sq Squa- 

 mosal. Ep Epiotic. Pr Petrosal. Sb Basi- 

 sphenoid. Ah Alisphenoid. Or S Orbito-sphenoid. 

 Fa Prefrontal. Fp Postfrontal. Fr Frontal. 

 Na Nasal. Ps Parasphenoid. Vo Vomer. Px Pre- 

 maxilla. gl Articular surface for the Hyoman- 

 dibular. Eth Ethmoidal cartilage, vag Foramen 

 for the exit of the vagus. 



part of the osseous ce- 

 phalic skeleton 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, O., Ueber das Zahnsystem der Amphibien. Arch. f. mikr. Anat. 

 Bd. XI. Supplement. 



§ 343. 



With regard to the various bones, we divide the primordial 

 cranium into the regions which we have already distinguished. The 

 occipital region is made up of four bones. The basi-occipital is a 

 direct continuation of the centra of the vertebras (Fig. 242, Ob). 

 In its hinder portion there is a cavity which is filled by the noto- 



2 g 2 



