HYOMANDIBULA OF THE GNATHOSTOME FISHES 579 
In one specimen of Mustelus (probaby laevis) a small carti- 
lage was found attached to the anterior edges of the articulating 
ends of the pharyngobranchial and epibranchial of the first bran- 
chial arch, this cartilage thus apparently being the interarcual 
cartilage that lies between the hyal and first branchial arches, 
and if so it is important in that it shows that these cartilages can 
be related either to the anterior or posterior one of the two 
arches between which they lie. No other independent interar- 
cual cartilage was found on either side of the head, but there was 
a somewhat corresponding process on the anterior edge of the | 
dorsal end of each epibranchial, similar to the processes shown 
by Parker (’76) in Seyllium canicula. 
These interarcual cartilages and the related ligaments of se- 
lachians thus have exactly the relations to the branchial clefts 
and inner cartilaginous arches that the ‘epitremal’ longitudinal 
processes of the cartilaginous branchial arches of Ammocoetes 
and Petromyzon have (Gaupp ’06), and if the cartilaginous arches 
themselves of these fishes are homologous, as seems so probable, 
it would seem as if the interarcual and epitremal cartilages must 
also be homologous, notwithstanding that the interarcual liga- 
ments of selachians lie internal to the efferent branchial arteries 
while the epitremal processes of Ammocoetes, as shown by Fa- 
varo (08), lie external to those arteries. 
The conditions found in the fishes above described, notwith- 
standing the limited number that were examined, seem. to war- 
rant the conclusion that the extrabranchials of the Plagiostomi 
—whether dorsal or ventral, and whatever their origin—have 
had their basal portions either developed or specialized in pro- 
tective relation to the related vena jugularis. And as these ex- 
trabranchial cartilages are presumably archaic structures, and 
as both dorsal and ventral ones are said to be found, in all the 
Plagiostomi, related to the hyal as well as to the branchial arches, 
there seems no reason to doubt, not only that they were developed 
also in the prehyal arch or arches of those fishes but also in the 
corresponding arches of the common ancestor of all the gnathos- 
tome fishes, and that, accordingly, rudiments or modifications 
of them should be found in the Teleostomi and Dipneusti. In 
JouRNAL or MorpHouogy, Vou. 26, No. 4, 
