5 SO MANDIBULAR AND HYOID BARS. 



tudinally into an anterior and a posterior part (fig. 334). The 

 former constitutes 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 ossified (fig. 335) as 

 the epiceratohyal (ep.Ji) above, then a large ceratohyal (c/t), 

 followed by a hypohyal (JiJi), while the median ventral element 

 forms the basi- or glossohyal (gJi). 



The hyomandibular itself is articulated with the skull below 

 the pterotic process (fig. 334, H.M}. Its upper element ossifies 

 as the hyomandibular (fig. 335, h.m.}, while its lower part (fig. 

 334, Sy), which is firmly connected with the mandibular arch, 

 ossifies as the symplectic (fig. 335, sy). A connecting element 

 between the two parts of the hyoid bar forms an interhyal (iJi). 



There are more important 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 independently developed bar of 

 cartilage, completes the upper arcade in front ; but even with 

 this bar the two halves of the upper branch of the arch do not 

 meet anteriorly, but are separated by the ends of the trabeculae. 

 The anterior bar of the upper arcade is known as the 

 palatine ; but it appears to me as yet uncertain how far it is to 

 be regarded as an element, primitively belonging to the upper 

 arcade of the mandibular arch, which has become secondarily 

 independent in its development ; or as an entirely distinct 

 structure which has no counterpart in the Elasmobranch upper 

 jaw. The latter view is adopted by Parker and Bridge, and a 

 cartilage attached to the hinder wall of the nasal capsule of 

 many Elasmobranchii is identified by them with the palatine rod 

 of the Teleostei. 



The arch itself is at first very similar to the succeeding 

 arches ; its dorsal extremity soon however becomes broadened, 

 and provided with an anteriorly directed process. This part (fig. 

 334, M.Pt and Qu] is then segmented from the lower region, 



