ON THE THEORY OF THE VERTEBRATE SKULL 557 



gated bone in the Lacertilia, and still more in the OpJiidia, in which 

 the quadratum is carried at its extremity.^ 



In the Amphibia the petrous and mastoid have the same relations 

 as in the Reptilia ; but it is interesting to remark, that in some 

 Amphibia the anterior margins of the petrosal encroach upon the 

 lateral walls of the skull so as completely to enclose the exit of the 

 trigeminal, just as the posterior margin of the alisphenoid encroached 

 so as to inclose it, in the sheep. It can be hardly necessary to 

 remark, however, that this result has nothing to do with the disappear- 

 ance of any element in the postero-lateral cranial walls, which have 

 the same composition in the frog as in the crocodile or lizard. 



The determination of the homologues of the squamosal, incudal, 

 Meckelian, and tympanic elements in the amphibian skull is by no 

 means an easy matter, but one requiring a much more careful in- 

 vestigation than it has yet received. 



In Mammalia, a second arch, the hyoid, is connected with the outer 

 surface of the skull, immediately behind the mandibular, and more 

 particularly with that of the mastoid bone or its rudiment. The 

 proximal end of this arch (which is, at first, like the mandibular 

 arcade, a simple cartilaginous rod), in fact, usually becomes continu- 

 ously ossified with the mastoid, forming part of the walls of the 

 styloid canal ; while below this, and external to the tympanum, it is 

 converted into that slender bone, which is known as the styloid 

 process. 



In adult birds and most reptiles, the upper end of the hyoid arch 

 is free, but in some Reptilia ^ it is attached by a styloid process to 

 the representative of the mastoid. Whether attached to the cranium 

 or not, in all abranchiate Vertebrata the proximal end of the hyoidean 

 arch is quite distinct from that of the mandibular arch. 



In the Amphibia, however, I find a condition of the proximal 

 ends of these two arches, which seems to foreshadow that intimate 

 connexion between them which obtains in fishes. On the outer side 

 ■of the petrosal, and of that part of the exoccipital which represents 

 the mastoid, there lies a cartilaginous mass, which is continued down- 

 wards into a pedicle, with whose lower end the mandible is articu- 

 lated. From the anterior edge of the proximal half of this pedicle, 

 the narrow cartilaginous basis of the pterygoid passes forwards and 



' See for the manner in which this is brought about, Rathke's ' Entwick. d. Natter.' 

 Rathke, it should be said, regards this bone as the tympaniciim, bat its primitive place and 

 mode of origin are those of the squamosal of the mammal. 



^ See Cuvier, ' Ossemens Fossiles,' x. p. 65 ; and Stannius, ' Zootomie der Amphibien,' 

 :p. 6S. 



