MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
71 1 
takes its place as the proximal element of the mandibular arch, is apposed by its upper 
extremity to its fellow in the median line beneath the basis cranii, while its lower 
end is applied to the posterior margin of the quadrate. In full-grown Sturgeons there 
are two ectosteal mesopterygoid plates, thicker than those in Polyodon, and applied 
to the inner and outer sides of the anterior ends of the two palato-quadrate bars, which 
are also in contact beneath the basis cranii. A massive pterygoid flanks the inner side 
of the axial cartilage, and a maxilla and a jugal the outer side. But a much more 
important difference between the two Ganoids is the presence of a palatine bone in 
Acipenser, apparently due to the ossification of the antero-lateral edge of the palato- 
quadrate cartilage, and of a quadrate, ossicle. In the Sturgeon there is no trace of 
the leaf-like process which grows forwards from the quadrate cartilage of Polyodon * 
The existence of this leaf-like outgrowth is one of the most interesting anatomical 
features possessed by this Ganoid. Its superficial relation to the levator mandib daris 
muscle which passes downwards between it and the main body of the pterygoid 
process to its insertion of the mandible, and its evident origin as an outgrowth from 
the proximal half of the first postoral arch at once suggests a comparison with the 
“orbitar process” of the Anurous Amphibia. In the Tadpoles of all the Anura whose 
cranial development has been worked out, a flap of cartilage grows out from the upper 
edge of the much-inclined mandibular pier, arches over the leva, tor mandibularis, and 
either terminates freely above, as in the Frog, or joins the prefrontal region of the 
trabecula, as in the Toad.t 
At a later stage, when the rotation backwards of the mandibular pier has caused it 
to take up a position nearly at right angles to the axis of the skull, the orbitar process, 
now much reduced in relative size, instead of being directed upwards, is now directed 
forwards, and appears as a styliform outgrowth from the hinder edge of the quadrate 
cartilage. It is this particular stage which is depicted in Plate 58, figs. 2 and 3, of 
Air. Parker’s memoir , \ and, though transitory in Dactylethra ccipensis, yet appears to 
be permanently retained by Polyodon .§ 
In the permanent retention of this embryonic feature, Polyodon is almost unique 
amongst Ichthyopsida, the Lamprey being the only fish in which the “orbitar process” 
has retained its probable primitive function of acting as a suspensor to the much- 
inclined suspensorium of Vertebrata possessing a suctorial mouth. This correlation of 
a suctorial mouth with the existence of a well-marked “ orbitar process,” and with a 
backward rotation of the suspensorium during the course of their cranial development, 
which is so marked a feature in the embryo Anura, at once suggests the probability 
* Vide Parker, “ Development of the Face in the Sturgeon,” ‘Monthly Microscopical Journal,’ 1873. 
t Parker, “ On the Structure and Development of the Skull in Bafrachia,” Part II., Plate 55, fig. 3, 
Phil. Trans., 1876. 
+ Loc dt., Phil. Trans , 1876. 
§ Mr. Parker informs me that he has found a distinct orbitar process in a half-grown OtihpJivs 
margaritifer and even in an adult Bufo ornatus. 
MDCCCLXXVIII. 4 Y 
