SKULL 83 



tipper jaw. In Teleosts, however, the vomers, and in Amphibians the 

 palatines also, are no longer situated on parts of palatoquadrate, 

 but are related to the ethmoidal skeleton, and lie beneath the 

 olfactory capsules. The pterygoid alone retains its original 

 relations to the palatine bar in Amphibians and many Reptiles. 



The palatoquadrate cartilage, which in Elasmobranchs is 

 situated along the upper margin of the mouth, represents the 

 primary upper jaw, and does not correspond to the premaxillo- 

 maxillary bar which lies externally to it in the higher Fishes and 

 in all Vertebrates above them. It is possible that the premaxillse 

 and maxillae were originally laid down in relation with certain of 

 the labial cartilages referred to above (p. 82, Figs. 63, 65, and 66.) 



In the lower jaw, investing and tooth-bearing bones are formed 

 around Meckel's cartilage. As in the upper jaw, two bars, an 

 outer and an inner, may be distinguished, the former represented 

 by the dentary and the latter by the splenud, in connection with 

 which there may be a varying number of presplenials. As the 

 teeth on the primordial lower jaw of Elasmobranchs may be looked 

 upon as corresponding to splenial teeth, it is possible that the 

 dentary. like the premaxilla and maxilla, may have been originally 

 formed around a primary skeletal element situated anteriorly to 

 the primitive jaw. 



The purely integumentary ossifications also of the lower jaw 

 are investing bones of Meckel's cartilage; : great confusion exists 

 as regards their nomenclature (" dermangular" " dermarticular" 

 " supra-angular" " coronary "). 



Certain tooth-bearing bones are formed in connection with the 

 hy< (branchial skeleton of Teleosts (superior and inferior pharyngcal 

 lows, " dcrmvbranchials" "dermewtofflossal"). In higher forms, 

 investing bones are only exceptionally present in connection with 

 this part of the primordial skeleton. 



Relations of the Replacing Bones. 



The following are usually looked upon as ossifications of the 

 occipital region (cf. Figs. 67-95) : the basioccipital, exoccipitals 

 (plcuroccipitals), and svpmoccipital : the last mentioned, however, 

 usually arises from the ossification of that part of the roof of the 

 skull (tedum synoticum, Fig. 60) which belongs to the auditory 

 region. The supraoccipitals and exoccipitals usually extend into 

 the auditory capsules, in each of which (otic region) arise the 

 periotic bones, including an opisthotic, epiotic, prootic, sphenotic, and 

 pterotic : of these, the most constant element is the prootic. The 

 sphenotic and pterotic occur only amongst Fishes, in which, how- 

 ever, the pterotic does not remain independent, but unites with a 

 dermal bone to form the squamosal, which is thus made up of an 

 autosquamosal and a dermosguamosal. 



G 2 



