GREGORY: FISH SKULLS 81 



Woskobojnikoff (1914, quoted by Sewertzoff, 1927, p. 479) held that the hyomandibular 

 of the sharks does not belong originally to the hyoid arch at all, but represents a dorsal 

 segment of the mandibular arch, hence a pharyngo-mandibular; but Sewertzoff (1927) 

 shows that in its relations to the muscles and cranial nerves the hyomandibular of sharks 

 corresponds to an epihyal, as held by Allis. 



As to the hyomandibular of teleostomes, Allis (1918) regarded it as a complex of two 

 parts; the anterior derived from the anterior branchial-ray bar of the hyal arch, and the 

 posterior derived from the dorsal extra-branchial of the hyal arch. This conclusion rested 

 in part on the fact that in the larval Polypterus there was a small and independent bit of 

 cartilage posterior to the articular head of the hyomandibular, and that the so-called 

 "accessory hyomandibular" of the adult Polypterus (see p. Ill below) was developed in 

 relation to this piece. Edgeworth (1926), however, found that in an earlier stage of devel- 

 opment the "small and independent bit of cartilage" was not an originally separate carti- 

 lage but was formed quite late in development and is due to a separation of a posterior 

 process of the head of the hyomandibular from the main part; also that the osseous "acces- 

 sory hyomandibular was a covering bone" {} of derm bone origin) and different in nature 

 from the hyomandibular itself. After comparing the development of the hyomandibular 

 of Jcipenser, Lepidosteus and Amia with that of Mustelus, Edgeworth concludes (1) that 

 the hyomandibular of teleostomes is a single structure and not the result of fusion of two 

 skeletal structures; (2) that in spite of the fact that in teleostomes the head of the hyo- 

 mandibular lies above the vena capitis lateralis, while in sharks it lies below it, there is no 

 constancy of position of the head of the hyomandibular in relation to the auditory capsule 

 in either sharks or teleosts and that the evidence indicates that the hyomandibular of 

 teleostomes is fully homologous with that of sharks. Goodrich (1930, p. 419) concludes 

 that "on the whole, for the usually accepted view that the hyomandibular is homologous 

 in all these fishes, there is good evidence not only from embryology but also from palaeon- 

 tology. . . . That the articular head [of the hyomandibular] is an 'otic process' is doubtful; 

 but it is not impossible that an articulation, originally ventral In Selachians, may have 

 moved up to a new position by passing over the bridge forming the outer wall of the jugular 

 canal into which the vein and nerve have sunk in Teleostomes (Stensio, 1921)." Sewertzoff 

 also from his embryological studies on sharks and teleostomes accepts the homology of the 

 hyomandibular in the two groups. Accordingly, as regards the disputed question of the 

 homologies of the hyomandibular in sharks and teleostomes, I adopt provisionally the view 

 that the hyomandibular of the teleostomes is truly homologous with that of the Selachii 

 (Edgeworth) but that it has shifted dorsally, passing over a groove and bridge containing 

 the vena capitis lateralis and part of the facial nerve (Stensio, 1921). 



The symplectic of teleostomes appears to represent only the lower part of the hyo- 

 mandibular, which in Polypterus is not yet separated off from the main part of the element. 



With regard to the branchial arches, Allis pointed out in several important papers 

 (1915, \923b, \92Sb) that in the gnathostome fishes as the mouth and branchial arches 

 enlarged, two distinctly different forms of branchial arch arose: first, the Sigma-shaped arch 

 of the Selachii in which the pharyngobranchials project postero-mesially, second, the V- 

 shaped arch of the Teleostomi in which the pharyngobranchials project antero-mesially. 

 Allis's theory (1925^) in brief is that the branchial clefts became prolonged dorso-anteriorly, 

 causing the reduction of the posterior projections of the pharyngobranchials and the pro- 



