DERMAL BONES OF FISHES. 137 



parietal, the mastoid, and the pre-opercular, as well as the turbinate, 

 the sub- orbital, and the supra-temporal bones, offer this modificatiou 

 of their outer surface. The same correspondence in the pattern of 

 the exterior markings usually prevails in all these bones, and is very 

 conspicuous in some fishes ; as in the bold net-work and deep depres- 

 sions of the surfiice, observable in the Pristipoma and some Sciajnoids ; 

 and in the entirely exposed, enamelled, and shagreened surface of 

 the same bones, together with the maxillary ai'ches, in the Pohjpterus. 

 This correspondence of exterior character, though it diminishes the 

 contrast between the endo- and exo-skeleton bones of the skull, does 

 not destroy their distinction. In certain parts of fishes the endo- and 

 exo-skeletons are so connected together that we can scarcely find the 

 boundary line in nature ; yet the advantage to the Osteologist of 

 classifying the multiform subjects of his study according to their 

 typical characters must not, therefore, be abandoned. 



Guided by the skull of the Lepidosiren, and by the light of the 

 general homology of the opercular bones as diverging appendages of 

 the tympano-mandibular arch, I consider the pre-opercular, sub- 

 opercular, and inter-opercular bones to be parts of the endo-skeleton. 

 The opercular bone is very constantly represented by the large 

 dermal plate in the Sturgeon, which M. Agassiz regards as being, with 

 the supra-scapular dermal plate, an anterior continuation of the 

 lateral series of dermal scales. There is also a small dermal plate 

 upon the opercular flap, below the large opercular plate, and which 

 small plate might be regarded as the horaologue of the sub-opercular 

 bone. All the four opercular bones forming the diverging appendage 

 of the tympano-mandibular arch Avere deemed by Cuvier to be 

 peculiar ichthyic super-additions to the ordinary vertebrate 

 skeleton ; whilst by Spix, Geoffrey, and De Blainville they are 

 held to be modifications of parts which exist in the endo-skeleton of 

 other Vertebrata. The learned Professor of Comparative Anatomy in 

 King's College, who regards this as " the more philosophical mode of 

 considering them," * has briefly stated the homologies proposed by 

 the supporters of this view, viz. that the opercular bones are gi- 

 gantic representatives of the ossicles of the car (Spix, GeofVnjy, 

 Dr. Grant f) : or that they ai'e dismemberments of the lower jaw 

 (De Blainville, Bojanus), — a view refuted by tlie discovery of 

 the complicated structure of the lower jaw in certain fishes, c. g. 

 Sudis, ^fi<J- 08.), wiiich likewise possess the opercular bones : thirdly, 

 that tliey are parts of the dermal skeleton ; in short, scales modified 



* Profi'ssor Ryinor .TniU's, Gcner.-il Outline of tlic AniiiKiI Kincrdom, Svo. ISll, 

 p. 50;). 



f Lectures, lAinal, Jan. 11. IS.'M, p. 57:>. ; Outlines of Conij). Anal. i>. CI, 



