HYOMANDIBULA OF THE GNATHOSTOME FISHES 613 
espécially true of these cartilages in Chlamydoselachus, where, in 
the one specimen of this fish that I have examined, I find three 
in place of the one described by Fiirbringer (’03). These carti- 
lages of Chlamydoselachus are quite certainly rudiments of the 
ordinary branchial rays of the mandibular arch, and hence not 
rudiments of the dorsal extrabranchial of that arch. They may, 
however, represent the little chain of cartilages that, in Tor- 
pedo, connects the spiracular cartilage of that fish with the hyo- 
mandibula. This whole subject needs further investigation, 
which my material does not at present permit. 
The eye-stalk, under my present interpretation of the mandib- 
ular cartilages, can not be the homologue of the processus as- 
cendens palatoquadrati of amphibians, as I have quite recently 
suggested (Allis ’14 a). It still, however, seems to me that it 
must be a cartilage related to the dorsal half of the premandib- 
ular arch, and its posteriorly directed position, and its contact 
with the ventral portion of the cranial wall ventral to the orbital 
sinus both seem to indicate that it is a pharyngeal element of 
its arch. It lies ventral to the nervus ophthalmicus profundus, 
which is its proper relation to that nerve if both it and the nerve 
belong to the premandibular arch; but its relations to the effer- 
ent pseudobranchial artery and to the superior and inferior di- 
visions of the nervus oculomotorius need explanation. Luther 
(09) has described two little muscles in Stegostoma which are 
said to arise from the cranial wall and, running forward, to be 
inserted on the eye stalk, which, if the stalk is a premandibular 
pharyngeal element, would strongly suggest interarcuales dor- 
sales muscles related to that arch. 
In the Chondrostei the hyomandibula articulates with the 
neurocranium by a single articular head, and this head appar- 
ently has exactly the relations to the vena jugularis and nervus 
hyomandibularis facialis that the anterior articular head of the 
teleostean hyomandibula has. The conditions, however, differ in 
that, in Polyodon, and hence probably in the other Chondrostei 
also, the vena jugularis and nervus hyomandibularis facialis 
traverse a canal in the cranial wall which is apparently not a 
trigemino-facialis chamber comparable to the one found in the 
