Fig. 335. Young Salmon of the first summer, about 2 inches long; side view of skull, excluding branchial arches. (From Parker.)
The palato-mandibular and hyoid tracts are detached from their proper situations, a line indicating the position where the hyomandibular is articulated beneath the pterotic ridge.
ol. olfactory fossa; c.tr. trabecular cornu; ula. ulb. 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; p.c. condyle for palatine cartilage; 5. trigeminal foramen; 7a. facial foramen; 8. foramen for glossopharyngeal and vagus nerves; mk. Meckelian cartilage; op.c. opercular condyle.
Bones: e.o. exoccipital; s.o. supraoccipital; e.p. epiotic; pt.o. pterotic; sp.o. sphenotic; op. opisthotic; pro. prootic; b.s. basisphenoid; al.s. alisphenoid; o.s. orbitosphenoid; l.e. ectethmoid or lateral ethmoid; pa. palatine; pg. pterygoid; m.pg. mesopterygoid; mt.pg. metapterygoid; qu. quadrate; ar. articular; h.m. hyomandibular; sy. symplectic; i.h. interhyal; ep.h. epiceratohyal; c.h. ceratohyal; h.h. hypohyal; g.h. glosso- or basihyal.
The hyoid arch, though largely ossified, undergoes a process of development very similar to that in Raja. It is formed as a simple cartilaginous bar, which soon becomes segmented longitudinally 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.h) above, then a large ceratohyal (ch), followed by a hypohyal (hh), while the median ventral element forms the basi- or glossohyal (g.h).
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 (ih).
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 trabeculæ.
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, and forms what may be called the pterygo-quadrate cartilage, though not completely homologous with the similarly named cartilage in Elasmobranchs; while the lower region forms the Meckelian cartilage (Mck), which has already grown inwards, so as to meet its fellow ventrally below the mouth. The whole arch becomes at the same time widely separated from the axial parts of the skull.
Nearly simultaneously with the first differentiation of the mandibular arch, a bar of cartilage—the palatine bar already spoken of—is formed on each side, below the eye, in front of the mouth. The dilated anterior extremity of this bar soon comes in contact with an anterior process of the trabeculæ, known as the ethmo-palatine process.
In a later stage the pterygoid end of the pterygo-quadrate cartilage unites with the distal end of the palatine bar ([fig. 334], Pl.Pt), and there is then formed a continuous cartilaginous arcade for the upper jaw, which is strikingly similar to the cartilaginous upper jaw of Elasmobranchii.