ON THE THEORY OF THE VERTEBRATE SKULL 563 



with the condyle-bearing crown of the arch, and with its anterior 

 crus or pedicle (P.Q.). It is inclined backwards and upwards, 

 and terminates close to the base of the skull in a free pointed 

 extremity. 



The posterior piece (S.Y.H.M.), on the other hand, has its broad 

 and narrow ends turned in the opposite direction. Distally, or below, 

 it is a slender cylindrical rod terminating in a rounded free extremity 

 behind, but close to, the condyle for the mandible ; above, it gradually 

 widens and becomes connected with the cranial walls. On its posterior 

 edge there is a convexity which articulates with the rudimentary 

 operculum, and below this it gives off a short styloid process, to 

 which the cartilaginous cornu of the hyoid is articulated. Thus 

 the cartilaginous arch, which stretches from the auditory capsule to 

 the ethmo-presphenoidal cartilage, consists, in reality, of two perfectly 

 distinct and separate portions — the anterior division V-shaped, having 

 its anterior crus fixed and its posterior crus free above ; the posterior, 

 styliform, parallel with the posterior leg of the V and free below. 

 The anterior division supports the mandibular cartilage, the posterior 

 the hyoidean cornu. 



As ossification takes place, that part of the anterior crus of the 

 V-shaped cartilage which is attached to the ethmo-presphenoidal 

 cartilage becomes ih^ palatine ; its angle becomes '^v&jugal; between 

 these two the transverse and pterygoidien (represented by only one 

 bone in Gasierosteus) are developed in and around the anterior crus : 

 the tympanal arises in the same way around the free end of the 

 posterior crus. Thus these bones constitute an assemblage which 

 is at first quite distinct from the other elements of the suspensorium, 

 and immediately supports the mandibular cartilage. 



The proximal end (H.M.) of the posterior styliform division 

 gradually becomes articulated with the cranial walls, and, ossifying, 

 is converted into the temporal. The distal cylindrical end (S.Y.) 

 becomes surrounded by an osseous sheath, which at first leaves its 

 distal end unenclosed.. The bone thus formed is the symplectique, 

 which is at first free, but eventually becomes enclosed within a sheath 

 furnished to it by ihejiigal, and so strengthens the union of the two 

 divisions of the arch already established by the junction of the 

 tympanal with the temporal. The symplectique and temporal do not 

 meet, but leave between them a cartilaginous space, whence the 

 supporting pedicle of the hyoid, which ossifies and becomes the 

 osselet .<<tyloide, arises. 



The operculum, suboperculum, interoperculum, and preoperculum 

 are not developed from the primitive cartilaginous arch, but make 



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