MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
695 
of these processes, this roof was a complete one. A partial floor is formed for each 
orbit by a rough irregular outgrowth of cartilage (o.f., figs. 3 and 4) from the inferior 
edge of its inner wall. There is no supraorbital ridge, the junction of the inner wall 
of the orbit with the cranial roof being gently rounded off. The aliethmoidal, 
aliseptal, and subnasal outgrowths have combined to form a cup-shaped nasal capsule, 
supported on a short, thick neck. The thick posterior wall of each capsule is 
slightly produced backwards over the orbit beneath the dermo-postfrontal bone, and 
the lower lip of the cup, besides being somewhat more prominent than the upper, is 
slightly emarginate. This emargination of the subnasal outgrowth corresponds in 
position to the laterally placed posterior narial aperture (fig. 3, p.n.) ; the anterior 
nares, though situated in front of the former, are also lateral in position ( a.n .). 
I could detect nothing suggestive of the existence of either a free or a coalesced 
antorbital or palatine process. The only possible representative of that element 
is the slightly produced inferior and external angle of the posterior wall of the 
nasal sac. The long and depressed cartilaginous prenasal rostrum is small relatively 
to its size when invested with its splints. The upper surface is gently convex from 
side to side, but its floor and sides are straight. It diminishes very gradually in 
height, but rapidly in width, towards its slightly expanded and flattened anterior ter- 
mination. As in Acipenser, the rostral axis is slightly tilted upwards, so as to make 
a widely open angle above with the cranial axis. The spatulate appearance of its 
anterior end, as seen in Plate 55, figs. 1 and 2, is not due to the configuration of the 
axial cartilage, but to the disposition of the investing membrane bones. The lateral 
margins of the extreme anterior end of the rostrum are slightly flattened out, as in 
Raid and Rhinchobatus, and, as in these genera, are suggestive of the presence of a 
pan of upper labial cartilages. The roof of the rostrum is complete, with the excep- 
tion of a small median fontanelle (figs. 5 and 6, a.m.f) situated just in front of the 
nasal sacs, and evidently corresponding to the much larger anterior median fontanelle 
of the Skate and Rhinchobatus. In general appearance the rostrum of Poiyodon is 
very like that of the last-named Elasmobranch, as will be seen on referring to the 
excellent figures of the latter given by Gegenbaub. in his ‘ U ntersuchungen zur 
vergleichenden Anatomic der Wirbelthiere ’ (plate iii., fig. 1; plate vi., fig. 3, and 
plate ix., fig. 2). The rostrum of Rhinchobatus, however, differs from that of Poiyodon 
m the presence of labial cartilages coalesced with its anterior end, in the larger size of 
its median fontanelle, and in the absence of a lamina perpendicularis separating the 
cavity of the rostrum from the cranial cavity. But, as previously suggested, it is not 
improbable that the winglike expansions of the anterior end of rostrum in the Ganoid 
may point to the presence of upper labial cartilages which have coalesced with it. 
Huxley," in describing the chondrocranium of the Sturgeon, refers to the coales- 
cence of the anterior vertebrae with each other and with the skull. He says : “ At 
this point there is in the cranio-spinal cartilage of both the Sturgeon and Spatularia a 
* ‘ Lectures on Vertebrate Skull,’ pp. 204, 205. 
4 u 
MDCCCLXXVIII. 
