MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
709 
whole of the dorsal segment of the arch, and is a thin laterally compressed lamina 
of cartilage with a nearly straight inner, and a curved outer margin, resembling a 
segment of a circle. Ossification has invaded the proximal part of the cartilage, and 
given rise to an hourglass-shaped osseous epibranchial, and from its upper and pos- 
terior angle a short pointed ray is given off, which is directed upwards and backwards. 
The ceratobranchial (c.b 1 .), which forms the greater part of the ventral segment 
resembles the epibranchial in shape ; there is a small semicircular ossification marked 
with concentric striae near its ventral end. The short, stout, and slightly curved 
hypobranchial is attached to the anterior end of first basibranchial, in close contact 
with the hypohyal. 
The second and third branchial arches, which differ but little from the first, are 
attached by their pharyngobranchial elements to the hinder edge of the cranial end of 
the epibranchial belonging to the arch in front, and not directly to the skull. The 
second epibranchial (epdr.) also possesses a small cartilaginous ray, similar to that 
attached to the first. These hypobranchials are also attached to the first basi- 
branchial element. 
The hypobranchial of the third arch has its ventral end laterally compressed and 
bifid, like the proximal end of a rib, and between the two processes and the basi- 
branchial to which they are applied, one of the lateral trunks of the aorta passes on its 
way to the gill filaments of the arch it supplies. The fourth arch consists of epi- 
branchials and ceratobranchials only, though the latter may also contain a hypo- 
branchial element. They possess ossifications similar to those in the corresponding 
elements of the preceding arches. The epibranchial is attached dorsally to the 
proximal end of the epibranchial of the third arch, and ventrally to the second 
basibranchial. The distal end of the fourth ceratobranchial forks into two processes, 
the anterior of which is applied to the junction of the first with the second basi- 
branchial, while the posterior one is appl i ed wholly to the latter ; and between the 
two forks another branchial artery passes. 
The fifth pair of ceratobranchials are wholly cartilaginous, and lie in the floor of 
pharynx ; above, they are attached by ligament to the antecedent ceratobranchials, 
and below to the posterior end of the third basibranchial. The three basibranchials 
are entirely cartilaginous. The first has attached to it the hypohyal, and the hypo- 
branchials of the next three arches ; the second completes the fourth arch, and the 
third is attached posteriorly to the fifth pair of ceratobranchials. The coalescence of 
the hypobranchial elements of the third and fourth arches with second basibranchial 
in Dcictylethra and Pipa suggests that possibly the second and third basibranchials 
of Polyodon may be the mesially coalesced hypobranchials of the fourth and fifth 
arches. But the existence of three basibranchial in Acipenser, and the fact that 
hypobranchial and pharyngobranchial segments are the most variable of the branchial 
elements, as is shown by their absence in such Urodele Amphibians as Siredon and 
Menobranchus, and by their suppression in the hinder branchial arches of most Fishes, 
