ON THE THEORY OF THE VERTEBRATE SKULL 569' 



connected with the cranium, Pisces and Amphibia, as in so many 

 other particulars, agree with one another, and differ from Reptilia and 

 Aves on the one hand, as much as they do from Mammalia on the 

 other. And the difference consists mainly, as might be anticipated,, 

 in the large development in the branchiate Vertebrata of a structure 

 which aborts in the abranchiate classes. A most interesting series of 

 modifications, all tending to approximate the ramus of the mandible 

 more closely to the skull,^ is observable as we pass from the fish to 

 the mammal. In the first, the two are separated by the hyoman- 

 dibular, the quadrate, and the articular elements, the first of which 

 becomes shortened in the Amphibia. In the oviparous abranchiate 

 Vertebrata the cranium and the ramus are separated only by the 

 quadratum and the articulare, the hyomandibulare having disap- 

 peared. Finally, in the mammal, the quadratum and the articulare 

 are applied to new functions, and the ramus comes into direct contact 

 with the cranium. 



The operculum, suboperculum, and interoperculum appear to me 

 to be specially piscine structures, having no unquestionable repre- 

 sentatives in the higher Vertebrata. Much might be said in favour 

 of the identification of the preoperculum with the tympanic bone ;. 

 but there are many arguments on the other side, and at present I 

 do not see my way to the formation of a definite conclusion on this 

 subject. 



In the preceding discussion of the structure of the osseous 

 vertebrate skull, I have desired to direct your attention, more 

 particularly, to the consideration of those fundamental bones, the 

 determination of whose homologues throughout the vertebrate series 

 is of the greatest importance for my present object. The presphenoid, 

 ethmoid, mastoid, and petrosal are the Malakhoff and the Redan of 

 the theory of the skull ; and if anatomists were once agreed about 

 their homologues, there would be comparatively little left to dispute 

 about. 



But besides the axial, inferolateral, and superior series of bones,, 

 there are other, less constant, elements of the cranial wall, forming a 

 discontinuous superolateral series. These are the epiotic, the squa- 

 mosal, the postfrontal, the prefrontal, and lacrymal bones. Of the 

 two first-named of these bones I have already spoken sufficiently. 

 The postfrontal exists only in Reptiles and Fishes, and is always 

 situated between the frontal, alisphenoid, petrosal, and squamosal — 



1 Of course in a morphological sense. "Whether they are more or less distant in actual 

 space, is not the question. 



