698 
MR. T. W. BRIDGE OR THE OSTEOLOGY OF POLYODON FOLIUM. 
cleft splits into two canals, a large canal opening at the spiracle, and a smaller one 
which perforates the otic cartilage and opens on the superior surface of the skull. 
The canal is quite external to the semicircular canals and vestibule, though it 
approaches nearest to the horizontal canal. The meaning of this spiracular diver- 
ticulum, at first sight, was not very clear. I was inclined to regard it as having some 
connexion with the remarkable fenestration of the infero -lateral face of the periotic 
capsule which occurs in the very early stages of the embryo Salmon, and which 
Parker"" has called the “ primordial fenestra ovalis,” from its analogy to the fenestra 
ovalis of the abranchiate Vertebrata. 
But further consideration suggested to me that the diverticulum in question might 
be nothing more than a result of the rapid growth of the cartilage of the ear-sacs 
round the upper part of the first visceral cleft, so as to constrict it off from the rest of 
the cleft which remains as the spiracular canal. 
On examining a fresh Sturgeon’s skull, I found a diverticulum of the spiracular 
passage precisely similar to, but relatively smaller than that described in Polyoclon, 
and, as in the latter, opening into a basin-shaped depression in the roof of the otic 
capsule. The inferior orifice was just in front of the hyomandibular, and behind the 
mandibular gill ; the upper part of the canal was filled with fat. 
Periotic Bones (Plates 55 and 57, figs. 3 and 7). — At first it seemed evident that 
the membrane bones previously described were the only ossifications developed in con- 
nexion with the chondrocranium of Polyoclon, but after a careful examination I 
detected a small circular ectostea! scale, marked with concentric lines of growth over- 
lying the posterior part of the horizontal semicircular canal. This scale must be a 
rudimentary representative of the pterotic ( pt.o .). Behind the latter and immediately 
in front of the Vagus foramen, there was a second small ectosteal scale, oval in shape 
and slightly larger than the first. From its relation to the ampulla of the posterior 
vertical semicircular canal, this ectosteal patch must represent an opisthotic element 
(op.o.).t These appear to be the only representatives of the otic bones of other Fishes. 
A careful investigation of the epiotic, jmootic, and sphenotic regions, as well as of the 
rest of the external surface of the chondrocranium, failed to reveal the existence of 
any other ectosteal or of endosteal centres. 
Cranial Foramina . — Certain foramina opening on the roof of the cranium have 
already been noticed, but there still remain those which perforate its lateral walls and 
serve for the transmission of the cranial nerves and vessels. A small foramen 
perforates the side walls of the broad posterior part of the rostrum, at about the same 
distance in front of the olfactory sacs as the anterior median fenestra (figs. 3 and 6, Z). 
In the absence of a supra-orbital ridge there is no antorbital foramen for the Ramus 
ophthalmicus superficialis which would appear to pass dorsad of the neck of each nasal 
* “ Structure and Development of the Skull in the Salmon,” Phil. Trans., 1872, p. 118. 
t The relations of the otic bones to the horizontal and posterior vertical semicircular canals is well 
shown in Plate 57, fig. 7. 
