MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
719 
a transition towards the structure of the same parts in the typical Elasmobrancliii. 
In nearly all of those characters Polyodon differs from all other living Ganoids, with 
the exception of Acipenser and its nearest living and fossil allies. The extent to 
which this close correspondence in structure between the chondrosteous Ganoids and 
the Elasmobrancliii bears on the presumed phylogenetic relationship of the former to 
the latter will be referred to subsequently. 
C. Comparison of Polyodon with the Higher Ganoids and with the Teleostei. 
Comparison of Polyodon with the more specialised Ganoids and with the Teleostei 
shows that the former resembles the latter in possessing cranial and pectoral splints, 
rudimentary periotic ossifications, and in the partial ossification of its visceral arches ; 
in the presence of opercular bones, in the division of the proximal half of the hyoid 
arch into hyomandibular and symplectic segments, and in presence of an interhyal 
element. I may also mention the absorption of the inner wall of the otic capsule, and 
the development of a lamina perpendicularis ; the absence of labial or extra-branchial 
cartilages with the doubtful exception of such as may be confluent with the anterior 
end of the produced prenasal rostrum. The non-lobate character of the pectoral fins, 
and the lateral position of the anterior and posterior nares, as well as the fact that 
the interspinous bones, though thrice segmented, are not more numerous than the 
segments of the axial skeleton, as indicated by the neural arches, may also be regarded 
as instances of specialisation in the same direction. 
Previous to discussing the relations of Polyodon to the Amphibia we may consider 
the bearing of the facts elucidated in preceding detailed comparison of the genus with 
the Elasmobranchii, and with the other Ganoids, on its systematic position. Several 
anatomists have attempted to give expression to the important structural differences 
between the two genera of Polyodon and Acipenser and the remaining recent and fossil 
Ganoids in various schemes of classification, and it will be advantageous to give a brief 
sketch of their views. 
Agassiz, 4 ' in his newly constituted order of the Ganoidei, included Acipenser and 
Polyodon in the family Acipenserides. 
Heinrich Muller,! in his classical paper on the anatomy of the recent Ganoids, 
classified them as follows : — 
a. Holostei. — Osseous vertebral column. 
Family I. Lepidosteini . . . Lepidosteus. 
Family II. Polypterini . . . Polypterus. 
fi. Chonclrostei. — Cartilaginous vertebral column and persistent notochord. 
Family III. Acipenserini . . . Acipenser ( Scaphirhynchus ). 
Family IV. Spatularite , . . Polyodon (Lacep.) ; Planirostra (Raf.). 
* Agassiz, Pois. Foss, 
t ‘ Leber den Bau und die Grenzen der Ganoiden.’ Abhandl. Ak. Wiss. Berlin, 1864. 
MDCCCLXXVIII, 4 Z 
