582 EDWARD PHELPS ALLIS, JR. 
in the Teleostomi, the arches were carried bodily forward and 
the sigma-form of arch was not acquired. But whatever the 
cause may have been of the sigma-form of the branchial arches 
in the Plagiostomi, it is certain, as will be later shown, that this 
form was also impressed upon the dorsal end, at least, of the car- 
tilaginous bar of the hyal arch of these fishes, and probably also 
upon that end of the bar in the mandibular arch. 
Parker (’76, p. 211) says that the pharyngobranchials of Seyl- 
lum canicula normally turn backward, ‘‘the opposite direction 
to that taken by the hyoid and mandibular arches,”’ but having 
great mobility, they ‘‘may turn forward:” and he adds that, in 
the branchial arches, he has ‘‘figured them both ways for illus- 
tration.” Unless Scyllium differs markedly from Chlamydose- 
lachus in that the dorsal ends of the pharyngobranchials are sus- 
pended loosely beneath the vertebral column, or not at all at- 
tached to it, this great mobility of these cartilages must be 
limited to the articulation with the epibranchial, and the pharyn- 
gobranchials could only be directed forward as the result of a 
shifting posteriorly of the entire branchial apparatus. And un- 
less Scyllium also differs markedly from both Stegostoma and 
Chlamydoselachus, as will be shown immediately below, the dor- 
sal end of the hyal arch is certainly directed backward and not 
forward. Furthermore, Haddon (’87, p. 208), in a figure said 
to be copied from Marshall, shows the pharyngobranchials of 
Scyllium canicula directed backward at such a marked angle to 
the epibranchials that it seems almost impossible that they could 
ever be directed forward, and Gegenbaur (72) shows them di- 
rected backward in Scyllium catulus. The conditions shown in 
Parker’s figure giving a lateral view of the skull and branchial 
arches of the adult Scyllium canicula must then be exceptional, 
and it is to be regretted that this particular figure has been re- 
produced and perpetuated in so'many text books (Parker and Bet- 
tany, Cambridge Natural History, Wiedersheim, ete.). 
The hyomandibula can now be considered, and it will be best 
to begin with the Selachii. 
Gegenbaur (’72, p. 175) considered the so-called hyomandib- 
ulae of the Selachii, Batoidei and Teleostei (and doubtless also 
