Miss Agnes Crane — On Certain Living and Fossil Fishes. 217 
Chalk, and the Polypterini, comprising only the living Polypterus and 
Calamoichthys of Africa, alone represent this once prevailing race of 
fislies at the present day. The genus Polypterus is remarkable for 
the unique arrangement of its sub-divided dorsal fin, and by the 
possession of a double cellular air bladder, which most nearly approxi- 
mates to the lungs of the Dipnoi. It has least structural afiinities 
with the Ccelacanths, its nearest allies in time, and is most closely 
zoologically related to the rhomboidal scaled Saurodipterines of the 
Devonian, from which it is separated by an enormous gulf of geolo- 
gical time, as no intermediate links have been discovered. 
In the notochordal Phaneropleurini we find forms which most 
closely resemble the acutely lobate finned Lepidosiren. The shape 
of the body, number, position, and structure of the fins, and all the 
elements of the internal skeleton, exactly foreshadow those of the 
mud-fishes. Like them Phaneropleuron was covered with thin 
cycloidal scales, through which the long and well-ossified ribs show so 
plainly in the fossil state as to suggest the name of the genus. The 
dentition, however, differs from that of Ceratodus and Lepidosiren, 
being composed of a row of short conical teeth in each jaw ; and in 
the absence of the grooved dental plates so characteristic of the true 
dipnoi, it is uncertain whether this family can be associated with the 
other members of that order. The chain of descent is carried on by 
the Ccelacanthini, the only fringe-finned ganoids occurring in the 
Mesozoic rocks. They can be traced up from Ccelacanthus in the 
Carbonifei'ous, through Holophagus in the Lias and Undina in the 
Oolites, up to Macropoma in the Chalk. The family is distinguished 
by cycloid scales, hollow fin supports, and a notochordal skeleton 
built on the same principle as that of the mud-fishes. In some 
genera the walls of the air bladder are ossified. This peculiarity, 
which was first suspected by Mantell, is especially remarkable in 
Undina and Macropoma. No fossil Crossopterygids have been dis- 
covered in Tertiary strata, but it is the opinion of Professor Huxley 
that, as the rhomboidal scaled Saurodipterines of the Devonian rocks 
are now represented by the living Polypterus, so the stiff-walled 
lungs of the Lepidosiren are the homologues of the ossified air 
bladder of the Ccelacanths, and thus that genus carried up the 
cycloidal brauch of the Crossopterygids to the present day. 
Such, in the abstract, is the life history of fishes, a dass character- 
ized, like other divisions of the animal kingdom, by the extinction of 
some groups after a brief existence, and by the persistent endurance 
of others through untold ages. In the few genera of living ganoids 
we have undoubtedly the surviving descendants of a numerous and 
powerful race which prevailed in the Devonian epoch, and by the 
discovery of fossil dipnoal forms the progenitors of Ceratodus and 
Lepidosiren, the Dipnoi are likewise proved to be of ancient lineage. 
The greater part of the existing piscine fauna, on the contrary, is 
shown to be of comparatively modern date. Moreover, in con- 
sidering the fact that the early fishes are remarkable from a combi- 
nation of diverse characteristics which subsequently become the dis- 
tinguishing peculiarities of distinct families, and of a higher order, 
