﻿Miss Agnes Crane — On Certain Living and Fossil Fishes. 215 



mentary strata, and, therefore, the non-representation of this lowest 

 form of ichthyic life in " the records of the rocks " becomes less 

 remarkable. Of the cartilaginous Marsipobranchii, comprising 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 highly organized 

 descendants. In 1856, Pander, in his magnificent work on the 

 Silurian and Devonian fishes of the Russian Baltic Provinces, gave 

 numerous 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 ^ 

 retains only three sjDecies 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 booklets, 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 cyclostomous 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 Elasmobrancliii, embraces the sharks, dog 

 fishes, rays, and Chimceroids. The first of these families has enjoyed 

 a long range 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 once numerous family now verging towards extinc- 

 tion. The ChimcBroids appeared first in the Devonian, and live on, 

 but the Eays 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 

 apjjear 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 Palaeozoic 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 ai'e all 

 dwellers in fresh water, occur in Africa, and four inhabit the lakes 

 1 Enc. Brit., vol. xvii., part i., 1859, art. Palseontology. 



