MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
703 
distal moiety, so that a short ascending process is produced which carries a transverse 
groove for the reception of the co-adapted articular end of the quadrate ; its angular 
margin is gently rounded. Towards the ligamentous symphysis the axial core thins 
away, and finally terminates in a stout, cylindrical, mento-Meckelian bone. 
A long dentary splint (cl.) is applied along the whole length of the outer side of the 
cartilage ; by the bending inwards of its upper and lower edges a groove is formed in 
which the axial cartilage and the mento-Meckelian ossicle lie. The splenial is a thin 
splint applied to the inner side of the proximal portion of the cartilage and reaches 
nearly to the articular end. There is no os articulare, nor any distinct angular, supra- 
angular, or coronary bones. Dr. Traquair* supplies a view of the mandible with 
its investing splints (plate vii., fig. 2). 
Though the mandibular arch has no cartilaginous or osseous metapterygoid, yet the 
latter is not without a representative. The spiracular opening leads by a short tube 
directly into the pharynx, and in front of it, in the place of the spiracular cartilage, 
a strong ligament passes from the smaller of the two parasphenoidal alge obliquely 
downwards and backwards beneath the spiracle to be attached to the anterior margin 
of the upper third of the hyomandibular. 
Along the spiracular margin of this ligament the pharyngeal mucous membrane 
is produced into a number of short branchial filaments with attached bases and free 
extremities. I could not detect any cartilaginous rays in these filaments nor any 
cartilage in the ligament. 
The gill filaments constitute a spiracular or mandibular gill, homologous with the 
structures so named in the Selachii ; and the ligament is as clearly the representative 
of the metapterygoid ligament of that group from which, how T ever, the former differs 
in being attached to the hyomandibular instead of to the quadrate cartilage. But it 
may be pointed out that in the Shark some at least of the fibres of the metapterygoid 
ligament are attached to the hyomandibular. 
In Acipenser, in consequence of the increased transverse width of the skull, a long 
spiracular canal connects the external opening with the pharynx, and along the 
anterior margin of the slit-like pharyngeal opening of this canal the mucous membrane 
is produced into a series of branchial filaments precisely as in Polyodon, though there 
is nothing in the Sturgeon comparable to a metapterygoid ligament — a fact which 
supports Mr. Parker’s conclusion, that this element is represented by the triangular 
cartilage which, with its fellow, forms the posterior part of the roof of the protrusible 
proboscis. 
I am not aware that the presence of spiracular gills has previously been noticed 
m any adult Vertebrata except the Elasmobranchii. Neither Amici, Polypterus, nor 
Lepidosteus possesses any traces of such structures. Hence, so far as this point is 
concerned, Acipenser and Polyodon resemble each other and the Elasmobranchii, and 
differ from the remainder of the Ganoidei and from all the Teleostei. 
MDOOCLXXVITT. 
* Loc. cit. 
4 x 
