415 



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 Fertebrata. 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 ver- 

 tebrate skull, I have desired to direct your attention, more parti- 

 cularly, to the consideration of those fundamental bones, the deter- 

 mination 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 

 the extent to which it is absolutely in contact with any one of these 

 bones varying. 



The prefrontal and lacrymal bones are always developed in or 

 upon that lateral process of the ethmosphenoidal plate, which gives 

 attachment externally to the palatopterygoid arch ; consequently they 

 lie at the anterolateral ends of the frontal, and have more or less 

 close relations with it, the ethmoid and the palatine bones. 



Finally, the nasal bones (or bone) never enter into the composition 



2 F2 



