Miss Agnes Crane — On Certain Living and Fossil Fis/ies. 215 
mentary strata, and, therefore. the non-representation of this lowest 
form of ichtliyuc life in “ the records of the rocks ” becomes less 
remarkable. Of tlie cartilaginous Marsipobranchii, comprisiug the 
hag fishes and lampreys, the horny teeth alone would be susceptible 
of preservation, and their absence has been commented on as nega- 
tiving the evidence of progressive development among fishes, as it is 
obvious the most simply constructed forms should appear first on 
the scene of life in Order to give place to their more higlily organized 
descendants. In 1856, Pander, in his magnificent work on the 
Silurian and Devonian fishes of the Russian Baltic Provinces, gave 
nnmerous figures of what he supposed to be the teeth of small 
sharks from the Lower Silurian rocks, but these so-termed conodonts 
have not been accepted as of true ichthyic origin. Professor Owen 1 
retains only three species as possibly the teeth of fishes, and is of 
opinion that the remainder might be either the Ornaments of crus- 
taceans, “ or the spines, or hooklets, or denticles of naked mollusks 
or annelides.” Great numbers of these “ cone teeth ” have recently 
been detected in Carboniferous strata, both in England and America, 
and it is suggested that they may be the teeth of eyclostomous fishes 
like the hags and lampreys, and thus be the representatives of the 
Marsipobranchii of the ancient Silurian seas. They seem most to 
resemble in shape and structure the teeth of the Myxinoids, in which 
the dentition is peculiar, being composed of one horny conical tooth 
situated in the roof of the mouth, with two serrated dental plates on 
the tongue. It has been objected that the teeth of living cyclos- 
tomous fishes are horny or chitonous, while the fossil cone teeth are 
calcareous, but this applies with equal force to the theory that they 
are the teeth of mollusks, as the modern shell fish have siliceous teeth. 
The piscine derivation of the conodonts is, however, still a debated 
question requiring careful investigation, as it would antedate the 
appearance of ichthyic life in geologic history ; but if it cannot be 
asserted that they are the teeth of fishes, neither as yet can it 
be positively proved that they are not. 
The next Order, the Elasmobranchii, embraces the sharks, dog 
fishes, rays, and Chimceroids. The first of these families has enjoyed 
a long ränge from the Upper Silurian epoch to the present day. and 
one genus seems to have varied but slightly, the Cestracion Phillipi 
or Port Jackson shark of Australia being a descendant of the ancient 
Cestracionts, a ouce numerous family liow verging towards extinc- 
tion. The Chimceroids appeared first in the Devonian, and live on, 
but the Rays were not represented until the Jurassic age. The 
Placoderms, as we have seen, enjoyed but a transient existence, 
dying out at the close of the Devonian, while the Teleostei or true 
bony fishes which so largely predominate at the present day did not 
appear on the scene of life until the formation of the Cretaceous 
rocks. Seven living genera alone survive of the Ganoidei which 
prevailed so numerously in Paheozoic times, and but one of these, 
the sturgeon, the least characteristic of the group, is found in 
European waters. Two of the six remaining forms, which are all 
dwellers in fresh water, occur in Africa, and four inhabit the lakes 
1 Euc. Brit., vol. xvii., part i., 1859, art. Palaeontology. 
