54 



the Lower Siluiian rocks, but these so termed conodonts have not 

 been accepted as of true ichthyic origin. *Professor Owen retains 

 only three species as possibly the teeth of fishes, and is of opinion 

 that the remainder might be either the ornaments of crustaceans, 

 " or tlie spines, booklets, or denticles of naked mollusks or 

 annelides." Great numbers of these " cone teeth " have recently 

 been detected in carlioniferoiis strata both in England and 

 America, and it is suggested that they may l)e the teeth of 

 cyclostomous fishes like the hags and lampreys, and thus be the 

 representatives of the Mnrsipo hranclm of the ancient Silurian 

 seas. They seem most to resemble in shape and stmcture the 

 teeth of the BIyxmoids, in which the dentition is peculiar, being 

 composed of one horny conical tooth situated in tlie roof of the 

 mouth, with two serrated dental plates on the tongue. It has 

 been objected that the teetli of living cyclostomous fishes are 

 horny or chitonous, while the fossil cone teeth are calcareous, but 

 tliis applies with equal force to the theorj- that tliey are the teeth 

 of mollusks, as the modem shell fish have siliceous teeth. The 

 piscine derivation of the conodonts is, however, still a debated 

 question reqiuring careful investigation, as it would antedate the 

 appearance of iclitliyic life in geologic history ; but if it cannot 

 lie asserted that they are the teeth of fishes, neither as yet can it 

 be positively proA'ed that they are not. 



The next order, the Ekismobranchii, embraces the sharks, dog 

 fishes, rays, and ChiiucerokU. 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 

 Cedracioii PhUrtppi or Port Jackson Shark of Australia, being a 

 descendant of the old time Cestracionts, a once numerous family 

 now verging towards extinction. The Ch'inueroids appeared first 

 in the Devonian, and live on, but the raj^s were not represented 

 imtil the Jurassic age. The Placoderms, as we have seen, enjoyed 

 but a transient existence, dying out at the close of the Devonian, 



* Enc. Brit., vol. xvii., part 1, Art Palaeontology. 



