THE SKULL. 487 



size, and arrangement in different types of Fishes ; but owing to 

 their linear disposition it is usually possible to find a certain num- 

 ber 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 considerations 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 \ 



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 sue i 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 types these bones are liable to 

 vary very greatly from the usual arrangement. 



Besides these bones there is usually present in the higher forms 

 a lacrymal bone on the anterior margin of the orbit derived from 

 one of a series of periorbital membrane bones frequently found in 

 Fishes. Various supraorbital and postorbital bones, etc. are also 

 frequently found in Lacertilia, etc. which are not impossibly phylo- 

 genetically independent of the membrane bones irdierited from Fishes ; 

 and may have been evolved as bony scutes in the subdermal tissue 

 of the papillae of the sauropsidan scales. 



The visceral arches of Fishes, especially of the Teleostei, are 

 usually provided with a series of membrane bones. In the true 

 branchial arches these take the form of dentigerous plates; but no 

 such plates are found in the Amphibia or Amniota. 



The opercular flap attached to the hyoid arch is usually supported 

 by a series of membrane bones, which attain their highest develop- 

 ment in the Teleostei. One of these bones, the prseopercular, is 

 very constant and is primitively attached along the outer edge of the 

 hyomandibular. It seems to be retained in Amphibia as a mem- 

 brane bone, overlapping the attachment of the quadrate and known 

 as the squamosal; though it is not impossible that this bone may 

 be derived from a superficial membrane bone, widely distributed in 

 Teleostei and Ganoids, which is known as the supra- temporal. In 

 Dipnoi the bone which appears to be clearly homologous with the 

 squamosal would seem from its position to belong to the series of 

 dorsal plates, and therefore to be the supra-temporal ; but it is re- 

 garded by Huxley (No. 446) as the pra',opercular^ 



In the Amniota the squamosal forms an integral part of the 

 osseous roof of the skull ; but in the Sauropsida it continues, as in 

 Amphibia, to be closely related to the quadrate. 



' For some interesting remarks on the aiTangement of these bones in Fishes, vide 

 Bridge, " On the Osteology of Polyodon folium." PhU. Tram., 1878. 



^ It is not impossible that the solution of the difficulty about the praeopercular is 

 to be found by supposing that the praeopercular as it exists in Teleostei is derived 

 from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly tlian 

 in Teleostei its primitive position. 



