GREGORY: FISH SKULLS 103 



In typical sharks, rays and chimaeroids the dentition is integrated from masses of small 

 units, clustered in groups around the borders of the Meckelian cartilage and on the primary 

 upper jaw. In the chimaeroids there are tritoral masses, moulded into incisive plates. 

 Thus there is a complete absence of the dentigerous dentary, palatine and pterygoid bones 

 which are so characteristic of the teleostomous fishes. Nor do the teeth ever attain the 

 same degree of regional differentiation which one sees, for example, in the sheepshead 

 {Archosargus) among teleosts. Moreover, in the more primitive elasmobranchs the indi- 

 vidual teeth are still recognizably homologous with the shagreen, or placoid scales, whereas 

 in the oldest teleostomes the serial homology of teeth with bony ganoid scales is evident only 

 on comparison of the histological elements involved. Elasmobranchs also lack differenti- 

 ated teeth in the pharynx or on the branchial arches, which are a conspicuous feature in 

 many teleostomes. 



The chimaeroids have been shown by Dean (1906) to form a peculiarly specialized 

 early side branch of the palaeozoic elasmobranchs and not to be in any phylogenetic sense 

 intermediate between the sharks and the teleostomes, notwithstanding the fact that they 

 have paralleled the latter in developing an opercular flap which in the side view conceals 

 all of the gill openings but one, and that they parallel the teleostomes in the dominance of 

 the eyes over the olfactory organs. The large size of the eyes causes a marked constriction 

 of the interorbital region, which contributes further to the list of teleostome-like characters. 



The chimaeroids are remarkable for their extreme specialization for durophagous diet, 

 since they have strong nibbling or biting plates in the front of the jaws and cutting or 

 crushing plates on the margins of the jaws and roof of the mouth, the patterns varying 

 greatly in the different families of Mesozoic and modern chimaeroids. In order to afford 

 a firm support for this powerful dentition the upper jaw is completely fused with the skull. 

 This arrangement has long been called "autostylic," but as that term more properly apv 

 plies to the different conditions observed in the Dipnoi, I proposed (1904) to call the arrange- 

 ment seen in the chimaeroids holostylic. According to Dean (1906) the hyomandibular is 

 represented by a small dorsal piece of the hyoid arch. It seems probable, however, that 

 the hyomandibular has been fused with the posterior border of the quadrate and that the 

 mode of suspension seen in the chimaeroids has been derived from the primitive amphistylic 

 condition, in which the palatoquadrate was attached to the otic region both by its own otic 

 process and by the hyomandibular (see Goodrich, 1909, pp. 170, 171). According to this 

 view the hyomandibular still functions as part of the suspensorium. 



The labial cartilages of chimaeroids obtain an unrivalled development. They and their 

 muscles are well described by Sewertzoff (1927), who believes that they represent pre-oral 

 segments of the visceral arch series. They appear to function as movable lips. 



As a result of his studies on the embryology of chimaeroids, Dean tried to derive the 

 group from some Palaeozoic relatives of the cestracionts. But Tate Regan (1910i, pp. 

 836-837) held that in the Holocephali (chimaeroids) the hyoid arch is essentially similar to 

 the succeeding branchial arches, that the pharyngohyal is well developed and the hyoman- 

 dibular is not attached to the cranium, and that the cestracionts are true Euselachii, inas- 

 much as their "hyoid arch is modified in connection with the suspension of the jaws; the 

 pharyngohyal is absent and the hyomandibular is articulated to the cranium." If, how- 

 ever, the hyomandibular has been fused with the quadrate and with the cranium, as held 

 by Goodrich, the supposed difference between the chimaeroids and the cestracionts would be 



