MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
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Till. The cartilaginous branchial arches, consisting of dorsal and ventral segments 
uniting at an acute angle, and possessing small epibranchial and ceratobranchial 
ossifications. The absence of a basihyal element, and the consequent union of the 
hypohyals with the anterior end of the first basibranchial piece. 
IX. The massive, unsegmented, unconstricted, and persistent notochord, extending 
into the basis cranii as far as the pituitary fossa, and surrounded by a relatively thick 
skeletogenous layer, which in Polyodon supports dorsally the cartilaginous neural 
arches, and ventrally the haemal arches that enclose the dorsal aorta, the latter being 
also produced on each side into the continuous ridge which alone represents the trans- 
verse processes and ribs of other Ichthyopsicla. The appearance of inferior arches in 
the caudal region gives rise to the formation of a second ventral canal lying beneath 
that in which the aorta is contained, and enclosing the azygous cardinal vein. In 
Acipenser the axial notochord and its sheath support well-ossified neural arches, trans- 
verse processes, and ribs. In both genera a curious flattened, fibrous band traverses 
the medio -ventral line of the notochord, projecting into the dorsal aorta, and presenting 
interesting analogies in its position and relations to the “sub-notochordal rod” of 
the developing Frog and Shark. In both Polyodon and Acipenser the elements of the 
anterior part of the vertebral column are almost indistinguishably fused with each 
other and with the cranium into a continuous cranio-spinal mass. 
X. The presence of rudimentary intercalated neural arches lying behind the fora- 
mina for the exit of the anterior roots of the spinal nerves, and below those for the 
passage of the posterior roots ; and also of interhsemal cartilages having a similar 
relation to the true hmmal arches. 
XI. Cartilaginous scapular arches that conform to the Selachian type, but which are 
accompanied by post-temporal, supraclavicular, postclavicular, clavicular, and inter- 
clavicular derm bones. 
XII. In the structure of the pectoral, ventral, anal, and caudal fins, Polyodon and 
Acipenser to some extent resemble the Selachii, and notably in that their fins possess 
a marginal fringe of small cubical cartilages, in the segmentation of their interspinous 
bones and cartilages, and also in the fact that the latter are less numerous than the 
fin-rays which they support. 
A. Comparison of Polyodon with Acipenser. 
Though so closely related to Polyodon, and agreeing with that genus in most of the 
anatomical features to which attention has been directed in the preceding paragraphs, 
the Sturgeon differs from the former in several points, of which the most important 
are — the ossification of the retral palatine outgrowths as palatine bones ; the repre- 
sentation of the proximal elements of the mandibular arches by large metapterygoid 
cartilages, which are so coadapted as to form the hinder part of the roof of the 
protrusible proboscidiform mouth, and not merely by metapterygoid ligaments as in 
