XX] SKULL 479 



knee-like piece the epibranchial. The main portion of the arch 

 is ossified by the great curved ceratobranchial which slopes down- 

 wards and forwards. Below this is the hypobranchial which is 

 joined to its fellow by the median copula piece, the basibranchial ; 

 this piece connects the ventral ends of all the arches together. The 

 seventh and last visceral arch (or fifth branchial) consists of a single 

 bone on each side lying in the ventral wall of the pharynx and bear- 

 ing teeth. Since the arches slope downwards and forwards, it comes 

 about that the teeth borne by this rudimentary arch bite against 

 the teeth borne by the pharyngobranchials of the preceding arches, 

 and most of the chewing of a Teleostean fish is done in this way by 

 the action of the constrictor muscles of the pharynga which bring 

 these two sets of teeth together. This last arch is frequently 

 spoken of as the inferior pharyngeal bone (Fig. 235). 



Besides the bones which we have described which eat into and 

 replace the primitive cartilage there are others termed dermal 

 bones, which are derived from the ossification of the dermis, and 

 these we must now describe. Since the dermis forms an undivided 

 covering for the whole head and is continuous with the lining of 

 the stomodaeum it is impossible to assign dermal bones to definite 

 regions of the skull such as cranium, sense capsules and visceral 

 arches. They are better classified as (a) roofing bones, (6) bones 

 of the lips and cheeks, and (c) bones of the roof of the buccal 

 cavity or stomodaeum. To this category would have to be added 

 in some archaic Osteichthyes, but not. in Teleostei, (d) bones of the 

 under side of the throat. Outside the region of the skull we find 

 in Teleostei a series of membrane bones covering the pectoral 

 girdle. 



The roofing bones of the Teleostean skull are a pair of 

 parietal bones lying at the sides of the supra-occipital and 

 covering two membranous window or posterior fontanelles in the 

 cartilaginous roof of the cranium. In front of these come a pair of 

 frontal bones which extend forwards above the interorbital 

 septum and join the mesethmoid, and lastly a pair of small curved 

 nasal bones lying at the sides of the mesethmoid above the nasal 

 capsule. The bones of the cheeks and lips in the Teleostean 

 skull are comparatively numerous. In the upper lip there is a 

 premaxillary bone in front bearing teeth, behind this a maxil- 

 lary bone usually toothless but bearing teeth in primitive Teleostei 

 like the Salmon, and occasionally a small jugal bone behind this 

 again where upper and lower lips meet. In the lower lip there is 



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