80 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



somewhat larger. Here also the ossified membranes around the mouth have become en- 

 larged and folded back over the hyoid arch, while the bony membrane on the latter has 

 likewise become enlarged and folded over laterally to the remaining gill-arches. In the 

 ostracoderms there is no evidence that either the mouth or the minute gill-openings were 

 supported hy jointed arches like those of sharks. But, as already noted, there was a con- 

 tinuous supporting tissue in the septa between the gill chambers. 



It is hardly necessary to review in detail the evidence for the now commonly accepted 

 conclusion that the primary jaws, or oromandibular arches of the gnathostome vertebrates 

 are serially homologous with the branchial arches. (See Goodrich, 1930, pp. 396-^23.) 

 Even in the adult shark (Fig. 4) the topographic correspondences of the jaws themselves 

 to the branchial arches and of the adductor mandibulae muscles and their nerves to the 

 flexor muscles of the branchial arches and their nerves, reinforce the embryological evidence 

 as recently set forth by Sewertzoff (1927). Sewertzoff (1927, Taf. 29, fig. 10) has shown 

 that in the embryo shark the labial cartilages which lie on either side of the oropharynx 

 also have vestigial pouches suggesting those of the gill-arches. These pre-oral cartilages 

 reach a high degree of functional elaboration in the chimaeroids, where they somewhat 

 suggest small jaws lateral to the main jaws (Sewertzoff, 1927, Taf. 31). According to 

 Sewertzoff, the mandibular arch of gnathostomes represents the third of the series; this, 

 together with the hyoid arch and five to seven normal gill-arches of sharks, would make 

 nine to eleven in all. The view of Ayers (1921) that the jaws of gnathostomes have arisen 

 from rod-like structures in the velum of Amphioxus does not appear very tenable in the 

 light of more direct evidence for the view summarized above. The palaeontological evi- 

 dence, illustrated especially in the acanthodian and cladoselachian sharks (Fig. 5), reinforces 

 the embryological and morphological evidence in favor of the strict serial homology of the 

 oromandibular arch with the hyoid and branchial arches. 



Very obscure and complex is the problem of the serial homologies of the dorsal seg- 

 ments of the mandibular, hyoid and branchial arches. AUis (1915, 1923^, 1925^) has 

 maintained that the trabeculse of the embryonic chondrocranium represent "premandibular 

 arches which have swung upward to fuse with the membranous brain case," a view origi- 

 nally proposed by Huxley (Goodrich, 1930, p. 238). Also that the polar cartilages which 

 connect the trabeculae with the parachordals represent the dorsal elements (pharyngo- 

 mandibulars) of the mandibular ardhes, of which the palatoquadrates represent the epiman- 

 dibulars while the Meckel's cartilages represent ceratomandibulars. But both Sewertzoff 

 (1928, p. 202) and Goodrich (1930, p. 238) agree that the trabecular and polar cartilages 

 are part of the axial skeleton or neurocranium. 



The Hyomandibular Problem 

 As to the hyomandibular, Allis (1915, \923b, 1925^) has shown that there are two rad- 

 ically different types: (a) the selachian type representing an epihyal, which is ventral to 

 the vena capitis lateralis and {b) the teleostome type, which is dorsal to that vein and dorsal 

 to the adductor hyomandibularis muscle (see Goodrich, 1930, Fig. 446). That the selachian 

 hyomandibular is an epihyal was also held by Luther (1909) who found in Stegostoma tigri- 

 num that the hyomandibular had every appearance of being in series with the epibranchials, 

 and that it. also carried a small dorsal cartilage corresponding exactly to the pharyngo- 

 branchials (see Sewertzoff, 1927, pp. 447, 521). 



