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MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
The membranous fold that grows backwards from the hyoid arch over the gill 
opening is strengthened and supported by two thin squamose plates. Of these, one 
is attached to the outer side of the cartilaginous epiphysis at the distal end of the 
hyomandibular, and the other to the outer side of the interhyal cartilage. The first, 
which represents the operculum of the Osseous Fish, is produced backwards into 
several diverging, transversely striated flexible bony rays. It has no articular attach- 
ment, but is simply applied to lower end of the hyomandibular and retained in its 
place by fibrous tissue. The second, though usually described as a branchiostegal ray, 
seems to me to correspond rather to the interoperculum of Ceratodus, and, like the 
operculum, breaks up into a number of thin flexible rays. These two branching 
parostoses attached to the ventral and dorsal halves of the hyoid arch very curiously 
similate the two groups of coalesced cartilaginous rays, which in Selachii are attached, 
one to the hyomandibular, and the other to the ceratohyal. Lying in a groove on the 
outer side of the hyomandibular, I found a small cartilaginous filament, which I 
imagine is a remnant of one of these rays. A flattened nodule occupies a similar 
position in the Sturgeon, and I have also detected in Lepidosteus a small ray attached 
to the synchondrosis between the hyomandibular and symplectic bones and applied to 
the inner side of the operculum. Huxley has described similar structures attached 
to the inner side of the operculum and interoperculum of Ceratodus and Lepidosiren. 
The coexistence of these cartilaginous rays, which are so characteristic of the Shark 
and Hays, with the simple or branched parostoses which eventually replace them 
in the Teleostei, is an interesting transitional feature in these Ganoids and Dipnoid. 
Branchiostegal rays may be regarded as the parostoseal representatives of such of 
the free cartilaginous rays as are attached to the lower end of the ceratohyal of the 
Shark. 
The opercular gill, which is well developed in the Sturgeon, is altogether absent in 
Polyodon. 
The Branchial Arches. (Plate, 57, fig. 9.) 
The five branchial arches are very curious structures. The lateral half of each complete 
arch consists of a dorsal and a ventral segment uniting with each other at a very acute 
angle. The dorsal segment, formed by the pharyngo branchial and epibranchial elements, 
is directed backwards, downwards, and outwards to its union with the ventral segment 
while the latter passes forwards and inwards to join the mesobranchial pieces. 
The first three arches possess the typical number of elements, viz., pharyngo- 
branchial, epibranchial, ceratobranchial, and hypobrancliials ; the fourth arch has 
lost its pbaryngobranchial and hypobranchial branchials, and the fifth retains only 
its ceratobranchial element. 
The first pharyngobranchial (jt.br 1 .) is a short thick piece of cartilage attached by 
its conical extremity to the margin of the parasphenoid in the angle between the 
smaller ala and the body of that splint. The epibranchial ( ep.b h) forms nearly the 
