HYOMANDIBULA OF THE GNATHOSTOME FISHES 583 
of all the other Teleostomi) to be strictly homologous cartilages, 
and he apparently considered the single cartilage to represent the 
entire dorsal half of the cartilaginous bar of the hyal arch and 
hence to be the serial homologue of the combined epal and 
pharyngeal elements of the branchial arches. 
Parker (’76, p. 199) says that the hyomandibula of the Selachii 
represents ‘‘the whole of the upper part of the hyoid arch;”’ 
that “‘ Here, then, the ‘pharyngohyal’ and the ‘epihyal’ are in one 
piece, and the ‘ceratohyal’ and ‘hypohyal’ are in one;’’ and (p. 
205) that the suspensorial part of the hyal arch ‘‘is morphologi- 
cally a whole ‘epihyal’ piece, with no pharyngohyal segment above, 
and articulating with a ‘ceratohyal’ from which no ‘hyophyal’ 
element has been cloven.’”’? This would certainly seem to mean 
that Parker considered the hyomandibula to contain the unseg- 
mented pharyngeal and epal elements of the arch, and I have 
hitherto always so considered it, but as he also says (p. 211) that 
the hyomandibula is the ‘counterpart’ of the epibranchials, and, 
in a@ later work (’82, p. 147), that ‘‘In the Dog-fish (Scyllium) 
the hyomandibula is evidently the serial homologue of the epi- 
branchials,”’ it is not clear just what his opinion was. The same 
uncertainty exists as to the branchial arches also, for after first 
saying (’76, p. 199) that these arches ‘‘break up into four pieces 
on each side, a ‘pharyngo-,’ ‘epi-,’ ‘cerato-,’ and ‘hypo-branchial’ 
element,” he later says (p. 211) that the pharyngobranchials 
chondrify separately and independently of the main bar. One 
thing, however, he very definitely states, without later contra- 
diction or qualification; that there is no separate pharyngeal ele- 
ment in the hyal arch (’76, p 211). 
Dohrn (’84) implies, if he does not definitely say, that the car- 
tilaginous bars of the branchial arches, in both the Selachii and 
the Batoidei, chondrify as a single piece; that these bars later 
segment transversely, at the middle of their lengths, forming so- 
called epibranchial and ceratobranchial elements; and that still 
later the pharyngobranchials and hypobranchials are segmented 
off, respectively, from the dorsal ends of the epibranchials and 
the ventral ends of the ceratobranchials. In the dorsal half of 
the hyal arch this second segmentation is said not to take place, 
