THE SKULL. 



All 



al.» sp.o 



pt.oe.p 



UK- 



development very similar to that in Raja. It is f«jrme(l as a simple 

 cartilaginous bar, which soon becomes segmented longitudinally 

 into an anterior and a posterior part (fig. .334), The former consti- 

 tutes the hyomandibular [H.M), while the latter, becoming more and 

 more separated from the hyomandibular, constitutes the hyoid arch 

 proper ; owing to the disappearance of the hyobranchial cleft, it 

 loses its primitive function, and serves on the one hand to support 

 the operculum covering the gills, and on the other to support the 

 tongue. It becomes segmented into a series of parts which are ossi- 

 fied (fig. 335) as the epiceratohyal {ep.h) above, then a large cerato- 

 hyal [ch), followed by a hypohyal (/?/<), while the median ventral 

 element forms the basi- or glossohyal {g.h). 



The hyomandibu- 

 lar itself is articulated 

 with the skull below 

 the pterotic process (fig. 

 334, H. M). Its upper 

 element ossifies as the 

 hyomandibular(fig.335, 

 /^.m), while its lower 

 part (fig. 334, >Sfj/), which 

 is firmly connected with 

 the mandibular arch, 

 ossifies as the sym- 

 plectic (fig. 335, sy). A 

 connecting element be- 

 tween the two parts of 

 the hyoid bar forms an 

 ^interhyal [ih). 



There are more im- 

 portant differences in 

 the development of the 

 mandibular arch in 

 Elasmobranchii and 

 the Salmon than in 

 that of the hyoid arch, 

 in that, instead of the 

 whole arcade of the 

 upper jaw being formed 

 from the mandibular 

 arch, a fresh element, 

 in the form of an in- 

 dependently developed 

 bar of cartilage, com- 

 pletes the upper arcade 

 in front ; but even with 

 this bar the two halves 

 of the upper branch of 



Fig. 335. Young S.vlmon of the first summer, 

 about 2 inches long ; side view of skull, excluding 

 BRANCHIAL ARCHES. (From Paiker. ) 



The palato-mandibular and hyoid tracts are de 

 tached from their proper situations, a line indicating 

 the position where the hyomandibular is articulated 

 beneath the pterotic ridge. 



ol. olfactory fossa ; c. tr. trabecular comu ; ul^. ul^\ 

 upper labial cartilages ; p.s. presphenoid tract ; t.cr. 

 tegmen cranii; s.o.b. supraorbital band; fo. superior 

 fontanelle ; n.c. notochord; b.o. basilar cartilage ; tr. 

 trabecula ; jj.c. condyle for palatine cartilage ; 5. tri- 

 geminal foramen ; 7«. facial foramen ; 8. foramen for 

 glossopharyngeal and vagus nerves ; vik. Meckelian 

 cartilage ; op.c. opercular condyle. 



Bones: ^-o. exoecipital; s.o. supraoccipital ; e.p. 

 epiotic ; pt.o. pterotic; ap.o. sphenotic ; op. opisthotic; 

 pro. prootic; b.s. basispheuoid ; al.s. alisphenoid; o.s. 

 orbitosphenoid ; I.e. ectethmoid or lateral ethmoid; 

 pa. palatine ; pg. pterygoid ; ni.pg. mesopterygoid ; 

 mt.pg. metapterygoid ; qii. quadrate ; ar. articular ; 

 h.m. hyomandibular; sy. symplectic; i.h. interhyal ; 

 ep.h. epiceratohyal; c.h. ceratohyal ; h.h. hypohyal; 

 (j.h. glosso- or basihyal. 



