THE ENDOSKELETON 147 



pair of nasals; behind these again, and between the eyes, is a 

 pair of frontals, often accompanied by prce- and post-frontals. 

 Behind these is a pair of parietals, and one or more supra- 

 occipitals. On the sides of the head, at about the level of the 

 parietals, are the squamosals, and around the eye are several 

 orbitals, distinguished as pre-, supra-, post-orbitals, etc. The 

 operculum, or gill-flap, which is present in these fishes, is cov- 

 ered and augmented by supra-, sub-, and pre-operculars. 



In short, to anticipate the history a little at this point, we 

 see in the dermal scutes the first appearance of the so-called 

 dermal bones of the skull which in later forms are to sink in 

 beneath the surface and become internal, thus coming into 

 close connection with the primordial skull and the osseous 

 elements derived from it. They are not all inherited by higher 

 forms exactly as they occur in the ganoid, the question of 

 their retention being based in each case upon their functional 

 importance. Thus, the opercular series, retained in the fish, 

 becomes lost with the reduction of the part which they cover ; 

 the orbital series is retained in part by reptiles, but becomes 

 lost in birds and mammals, with the single exception of one of 

 the prae-orbitals, which becomes the lacrimal; and the supra- 

 occipital series becomes reduced to a single piece. On the 

 other hand, certain ones are retained in all higher vertebrates, 

 and are recognizable throughout, although by secondary fu- 

 sions and divisions they are not always strictly homologous. 

 Thus, the frontals may or may not include the originally 

 separate prae- and post-frontals, and in a given case the ab- 

 sence of one of these latter elements as a distinct piece may 

 mean either that it has fused with one of the others or has 

 been gradually reduced in size until it has become lost. The 

 frontals in some form, however, are among the most constant 

 of dermal elements, and the same may be said of the parietals, 

 squamosals and nasals, which can be traced in all the verte- 

 brate classes (Fig. 19). The ventral side of the cranium 

 becomes also encased in a similar manner by dermal bones that 

 develop in the roof of the mouth, among which are the vomers, 

 the palatines, the pterygoids, and the extensive parabasal 



