684 
MR. T. W. BRIDGE OR THE OSTEOLOGY OF POLYODOR FOLIUM. 
merit of the tin rays, and the disposition of the fulcra and rhomboidal scales ; 
and also discusses the relationship of Polyodon to the Palseoniscidee. 
VI. Burt G. Wilder (“ Notes on the North American Ganoids : Amia, Lepidosteus, 
Acipenser, and Polyodon.’' Proc. Am. Ass. for Advancement of Science, 
1875 and 1876) has described the brain of this genus. 
VII. Gegenbaur has described and figured the shoulder-girdle and pectoral fin 
in his ‘ Untersuchungen zur vergleichenden Anatomie der Wirbelthiere.’ 
Parts I. and II. 1864. 
VIII. Thacher (“Ventral Fins of Ganoids,” Trans. Connecticut Academy, vol. iv., 
1877) describes and figures the structure of the ventral fins of Polyodon. 
In so far as the above-mentioned memoirs refer to the structure of the cranium in 
Polyodon, I have ventured in this paper to supplement them by giving a more 
complete description of the skull accompanied by figures. I have also attempted to 
compare in detail the skull of Polyodon with the skulls of Teleostei, Elasmobranchs, 
Amphibia, and with that of its nearest living ally, the Sturgeon ; and in addition, I have 
endeavoured, roughly and tentatively, to point out the bearing of the facts elucidated 
on the phylogenetic relationship of Polyodon to the Elasmobranchs on the one hand, 
and to the remaining Ganoids and to the Amphibia on the other. 
The .destination of my specimen, which belonged to the Museum of Comparative 
Anatomy of the University of Cambridge, prevented me from investigating its histo- 
logical structure, and particularly the minute anatomy of the vertebral column. 
Of the various papers and memoirs on the structure of the skull in the Ichthyopsida 
with which I am acquainted, I must acknowledge my especial indebtedness to Professor 
Huxley’s articles on the “Amphibia” (Encyc. Britannica, vol. i. ), and on Ceratodus 
fosteri (Proc. Zool. Soc., 1876); to Dr. Traquair’s previously cited memoir on the 
Palseoniscidee ; and also to Professor W. K. Parker, not only for the numerous memoirs 
with which he has enriched this department of anatomical science, but for many sugges- 
tions, and for the information which he has given me about results as yet unpublished. 
My thanks are also due to my friend Mr. A. C. Haddon, Scholar of Christ’s 
College, Cambridge, for the drawings from which the annexed plates were taken. 
Membrane bones of the Cranium. (Plate 55, figs. 1, 2, 3, and Plate 56, fig. 4.) 
The cranial membrane bones of Polyodon are thin, narrow, and much elongated, 
studded with small pointed Ganoid tubercles which radiate from the comparatively 
thick central portion of each splint towards the thinner and almost fibrous extremities. 
Their sutural margins are so closely interwoven that it is almost impossible to separate 
contiguous splints from one another, and in their regular and symmetrical arrangement 
they present a striking contrast to the dermal plates of the Sturgeon. A pair of 
mesially opposed splints (Plate 55, fig. 1 , b l ) overlie the supraoccipital region, and 
extend backwards for a considerable distance dorsad of the coalesced anterior vetebrse, 
terminating behind m a pointed process. Though constricted in the centre, they expand 
