164 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



these bones on either side, a dentary in front, a splenial far- 

 ther back on the inner side, and an angulare extending forward 

 from the angle of the jaw to meet the other two. In addition, 

 a supraangulare is sometimes present behind the articulation of 

 the lower jaw with the quadrate. 



Several of these membrane bones may bear teeth. When 

 teeth are present they almost universally occur on the premaxil- 

 laries, maxillaries, and dentary ; but they may also occur on the 

 vomers, palatines, parasphenoid, and splenials, and occasionally 

 on the pterygoids. 



This leads to the question of the phylogenetic origin of these 

 membrane bones of the skull. All the evidence goes to show 



that not only these teeth- 

 bearing bones, but most of 

 the covering bones of the 

 skull, have arisen from the 

 fusion of dermal plates, 

 much like the placoid scales 

 of the elasmobranchs. In 

 the jaws the enamel-capped 

 spines have given rise to 

 the teeth, while the basal 

 plates, fusing together, form 

 the bones themselves. In 

 many forms this origin of 



the bones by the fusion of the bases of the teeth can readily be 

 seen (Fig. 173). In the covering bones of the cranium the 

 dental portion has disappeared. The remaining membrane 

 bones have arisen around the canals of the lateral line system, 

 the suborbital chain of bones being the most constant of these. 

 Through the walls of the skull formed by these cartilage 

 and membrane bones are foramina for the passage of nerves ; 

 and these openings afford important landmarks for the identi- 

 fication of certain bones, especially in those numerous cases 

 where different elements fuse together. The optic nerve passes 

 through the orbitosphenoid. Between the orbitosphenoid and 

 alisphenoid is an opening (sphenoidal fissure or foramen lacerus 

 anterior) through which pass the third, fourth, and sixth, and 





FIG. 173. Development of dermal (maxil- 

 lary) bone in Amblystoma by fusion of the 

 bases of teeth. b, bone; c, cartilage; d, 

 dentine of tooth ; e, epidermis ; /, tooth. 



