710 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
seems to negative this view. It seems much more probable that the fourth and fifth 
ceratobranchials each contain an unsegmented hypobranchial element. 
Comparison of the branchial arches of Polyodon with those of Acipenser does not 
reveal any important differences. In the latter the arches are more massive and 
rounded. The same elements are ossified, but ossification has more largely invaded 
the cartilage, so that the resultant bones are shaft-like and massive. 
There are three basibranchials as in Polyodon, but the second is rudimentary, being 
reduced to a small nodule of cartilage imbedded in the ligament by which the hypo- 
branchials of the third arch are connected together. The third and fourth hypo- 
branchials have their ventral ends split for the transmission of a branchial artery. 
The aorta, lying ventrad of the basibranchial pieces, gives off a large lateral trunk for 
the supply of the gills in relation with the third, fourth, and fifth arches, which passes 
outwards, downwards, and backwards, between the forks of the third hypobranchial, 
and after giving off a branch for the third arch, passes backwards between the forks of 
the fourth hypobranchial to the convex side of the fourth arch. 
The cartilaginous ray, which in Polyodon is attached to the second epibranchial, is 
represented in Acipenser by a cartilage much larger that the pharyngo-branchial, and 
is applied to the lateral wall of the skull immediately over the course of the jugular 
vein. The next arch has no such ray. These free rays in the Chondrosteous Ganoids 
correspond to the autogenous processes which are given off from the pharyngo- 
branchials and epibranchials of the second, third, and fourth arches of the Salmon. 
The splitting of the distal ends of the third and fourth hypobranchials, though I 
am not aware that it occurs in any Teleostei, is the rule amongst Ganoids. In Amia 
and Lepidosteous, the same branchial elements are similarly bifurcated, and I believe 
that such is also the case in Polypterus. 
Thus it will be seen that, though conforming mainly to the Teleostean type, the 
branchial arches of Polyodon and Acipenser in possessing free rays, in the forking of 
the hypobranchials, and in the non-coalescence of the dorsal elements of the posterior 
arches, differ from those of most Teleostei. The almost entirely cartilaginous nature 
of the arches, and the inclination to each other of the dorsal and ventral segments, are 
interesting embryonic and Chimaeroid features. 
The bifurcation of the proximal ends of certain of the branchial arches in Polyodon 
(and in many Teleostei also), is an interesting survival of a feature which is transitory 
in the hyoid arch of the Shark and Skate, though permanent in the mandibular arch 
of Amphibia. Each free ray may be compared to free “ otic process.” 
As has been already mentioned, the branchial and hyoid arches of Polyodon differ 
but very little from those of the Sturgeon, but as regards the mandibular arch the 
differences between the two genera are more marked, though mainly dependent on the 
adaptation of the osseous and cartilaginous elements of the palato-quadrate arcade to 
form the remarkable pro bosci diform mouth of the latter. The metapterygoid liga- 
ment is absent in the Sturgeon, and the broad triangular plate of cartilage, which 
