592 MEMBRANE BONES. 



Membrane bones and ossifications of the cranium. 



The membrane bones of the skull may be divided into two 

 classes, viz. (i) those derived from dermal osseous plates, which 

 as explained above (p. 542) are primitively formed by the coal- 

 escence of the osseous plates of scales ; and (2) those formed by 

 the coalescence of the osseous plates of teeth lining the oral 

 cavity. Some of the bones sheathing the edge of the mouth 

 have been formed partly by the one process and partly by the 

 other. 



In the Fishes there are found all grades of transition between 

 simple dermal scutes, and true subdermal osseous plates forming 

 an integral part of the internal skeleton. Dermal scutes are best 

 represented in Acipenser and some Siluroid Fishes. 



Where the membrane bones still retain the character of dermal 

 plates, those on the dorsal surface of the cranium are usually 

 arranged in a series of longitudinal rows, continuing in the region 

 of the head the rows of dermal scutes of the trunk ; while the 

 remaining cranial scutes are connected with the visceral arches. 

 The dermal bones on the dorsal surface of the head are very 

 different in number, size, and arrangement in different types of 

 Fishes ; but owing to their linear disposition it is usually possible to 

 find a certain number both of the paired and unpaired bones 

 which have a similar situation in the different forms. These 

 usually receive the same names, but both from general consider- 

 ations as to their origin, as well as from a comparison of different 

 species, it appears to me probable that there is no real homology 

 between these bones in different species, but only a kind of general 

 correspondence 1 . 



It is not in fact till we get to the types above the Fishes that 

 we can find a series of homologous dorsal membrane bones 

 covering the roof of the skull. In these types three paired sets 

 of such bones are usually present, viz, from behind forwards the 

 parietals, frontals and nasals, the latter bounding the posterior 

 surface of the external nasal opening. Even in the higher 



1 For some interesting remarks on the arrangement of these bones in Fishes, vide 

 Bridge, "On the Osteology of Polyodon folium." Phil. Trans., 1878. 



