HYOMANDIBULA OF THE GNATHOSTOME FISHES 611 
palatoquadrate and the cranial wall; in certain other fishes (Ho- 
lostei, Teleostei) fused only with the cranial wall; and in embryos 
of the Amphibia fused only with the palatoquadrate. 
In the Plagiostomi the pharyngeal element of the mandibular 
arch, as an independent cartilaginous element, does not exist, 
and the tissues representing it have probably been dispersed and 
utilized to form adjacent portions of the neurocranium (Allis 
’14b). It may perhaps be in part represented in the low ridge 
described by me (Allis’14 a) on the dorsal edge of the palatoquad- 
rate of Chlamydoselachus and there considered by me to be the 
homologue of the processus basalis palatoquadrati of the Am- 
phibia; but it seems to me much more probable that this ridge 
simply represents the dorsal edge of the quadrate, which is the 
epal element of the arch. The low ridge in Chlamydoselachus 
can nevertheless properly be called the processus basalis, for 
even in the Amphibia this process may, as in Rana fusea (Gaupp), 
develop in part from cells related to the quadrate and in part 
from cells wholly independent of that cartilage, these latter cells 
evidently representing the pharyngeal element of the arch. 
The extrabranchial of the mandibular arch has, in contradis- 
tinction to the pharyngeal element, quite certainly been pre- 
served, in the Plagiostomi, in the frequently largely developed so- 
called spiracular cartilage of the Batoidei, as is evident from a 
consideration of Gegenbaur’s (’72) and Parker’s (’76) several 
figures of these fishes. This spiracular cartilage of the Batoidei 
lies definitely lateral to the vena jugularis; its ventral end is at- 
tached, either by ligament or by a chain of cartilaginous rods, to 
both the palatoquadrate and the anterior edge of the hyoman- 
dibula; its dorsal end is either attached by ligament to the cran- 
ial wall dorsal, to the vena jugularis (Raia radiata, Allis), or ar- 
ticulates there with that wall (Raia clavata, Parker); and the 
nervus hyomandibularis facialis issues posterior, and the nervus 
trigeminus anterior to it. 
Dohrn (’85) thought this spiracular cartilage of the Batoidei 
could not have been derived from a branchial ray or rays of the 
mandibular arch, and one of his reasons in support of this view is 
that the spiracular cartilage lies anterior to the entire blood-ves- 
JOURNAL OF MORPHOLOGY, VOL. 26, No. 4 
