ME. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
707 
upper jaw, that the palatine elements may have coalesced behind with pterygoid 
outgrowths and then united with each other beneath the basis cranii, has nothing to 
recommend its acceptance. In view of the strong resemblance in the structure of 
then' jaws between Polyodon and Selachians, nothing short of actual demonstration 
can render this alternative explanation in any way tenable. 
The ITyoid Arch.' 1 ' 
The hyomandibular ( h.m .) is a long, relatively slender, well ossified shaft-like bone, 
with a vertically expanded proximal end, a much constricted central portion, and a 
vertically compressed distal extremity. Its axis is inclined backwards at an angle 
of less than 30° with the cranio-spinal cartilage. The cartilage-tipped cranial end of 
the bone fits into the vertical groove on the external face of the otic capsule dorsad of 
the horizontal semicircular canal, as in Teleostei, and not ventrad as in Elasmobranchs. 
The symplectic ( sym .) is a stout triangular piece of cartilage attached by its broad apex 
to the anterior angle of the distal end of the hyomandibular, and by its base to the 
lower part of the rounded posterior margin of the cpiadrate to which it is firmly 
attached by the two symplectic ligaments, while its long axis is directed forward 
at an angle of about 90° with that of the hyomandibular. A line drawn through 
the proximal end of the latter and the distal end of the former will make an angle 
of at least 135° with the fore part of the skull. It is this extraordinary inclination 
of the hyomandibular and the quadrate that carries backwards the articular end of 
the mandible to a point opposite to the foramina for the passage of the fourth or 
fifth pairs of spinal nerves, thus giving a backward extension to the gape which is far 
greater than that of any other living Vertebrate animal, approaching, nearest in this 
respect, to the amphistylic Selachians and autostylic Anurous Amphibians, but re- 
ceding from the other extreme as exhibited in the relatively short gapes and 
forwardly-inclined suspensoria of the Teleostei. 
The remainder of the hyoid arch consists of a stout curved interhyal segment (i.hy.) 
attached to the posterior margin of the symplectic near its distal end, a long and 
comparatively slender epi-ceratohyal segment, and an unossified hypohyal element 
which connects the arch to the anterior end of the first basibranchial. The only ossi- 
fication in the distal half of this arch is an hourglass-shaped ceratohyal ( c.hy .). The 
hyoid arch of Acipenser is essentially the same as that just described. In the seg- 
mentation of the proximal half of the arch into hyomandibular and symplectic 
pieces, and in the presence of an interhyal, Polyodon resembles the Teleostei, but 
resembles the Selachii in the non-segmentation of the epihyal from the ceratohyal, 
and in the attachment of the ventral half of the arch almost to the distal end of the 
symplectic ; in the possession of hypohyals and absence of a basihyal, Polyodon 
resembles the Skate, but in the attachment of the former to the first basibranchial 
element approaches such Urodele Amphibians as the Axolotl and Menobranchus. 
* See also plate vii., fig. 1, in Dr. Traquair’s memoir on the Palasoniscidaa, previously cited. 
