72 VERTEBRATE SKELETON 



dence that the mentomeckehan is derived from the anterior labial 

 of Elasmobranchs, p. 68.) 



The cartilage elements of the hyale, in Ichthyopsida, ossify as 

 bones which usually have the same names as the cartilages (p. 66). 

 For the parts in the higher groups reference must be had to the accounts 

 of the separate classes {infra). In most fishes the hyomandibular 

 cartilage gives rise to two bones (fig. 74), a hyomandibula which 

 usually articulates with the otic region of the cranium, and below this 

 a S3miplectic, which connects with the quadrate; hyomandibula, 

 quadrate and symplectic forming the suspensor of the lower jaw. 

 Above the fishes the fate of the hyomandibula is one of the unsolved 

 problems, one possibility being that it is represented by the stapes, 

 one of the bones of the ear. The branchial arches are greatly modi- 

 fied or degenerate in higher Vertebrates, less so in Ichthyopsida. In 

 so far as they ossify, the resulting bones bear the same names as the 

 cartilages from which they come (p. 65). 



Membrane Bones.- — The membrane bones of the skull are 

 regarded as derivatives of the skin, arising from scales or teeth, and 

 here the term dermal bones is most applicable, for in some cases, like 

 the sturgeons, there can be little doubt that at least a part of the 

 cranial bones come from the corium of the skin. Farther back in 

 history these bones are believed to have arisen by enlargement or 

 fusion of the bases of placoid scales or their homologues, the teeth 

 (fig. 6). On this supposition it is evident that the number of indi- 

 vidual bones would be greater in lower than in higher groups. For 

 our purposes a schematic skull (much like that of a Stegocephal) in 

 which numerous bones occur, afifords a good basis for locating the 

 cranial membrane bones (lig. 75). 



In all Vertebrates above Elasmobranchs membrane bones form 

 the roof of the brain case, the cartilage roof being greatly reduced 

 or absent. These roofing bones are primitively in pairs, one on either 

 side of the median line. In most Vertebrates the most posterior of 

 these roofing bones is single in the adult and is known as the inter- 

 parietal, which lies just in front of or above the supraoccipital. In 

 some lower forms and in the ontogeny of the higher two bones 

 (dermoccipitals, fig. 76) may occupy its place. In higher groups the 

 interparietal is often fused (as in man) with the supraoccipital, 

 sometimes with the parietals. Next in front is the pair of parietal 

 bones, with frequently, as in many reptiles and Stegocephals, a 



