Prof. Owen on the Structure of the Teeth of some Fossil Fish. 41 



in bundles. At first the frond contains only one or two fila- 

 ments (as correctly stated by Mr. H assail in his ' British Fresh- 

 water Algse ') ; but these dividing as in Oscillatoriay the inflated 

 frond becomes completely filled and at length ruptured, when 

 the filaments escape from it to form new plants. 



I intend in a future communication to offer some evidence in 

 proof that the appositional branches in Rivularia, Calothrix and 

 other genera are merely modifications of the mode of growth here 

 described. 



VII. — On the Structure of the Teeth of some Fossil Fish of the 

 Carboniferous Period. By Prof. Owen, F.R.S. 



To the Editors of the Annals of Natural History. 



Gentlemen, 

 In the interesting and instructive summary of the modifications 

 of the teeth in fossil fishes of the carboniferous period which 

 Mr. M'Coy has given in the ^Proceedings of the Cambridge 

 Philosophical Society,^ June 1848, he notices a layer of true 

 enamel in ' Centrodus/ which he says " is quite distinct from 

 that dense modification of dentine, which, forming the polished 

 surface of most fish-teeth, has been confounded with true enamel, 

 but which it is here proposed to call ^ ganoine ' in future descrip- 

 tions^' (p. 65). I have long been in the habit of applying the 

 term ' ganoine ' to the peculiar tissue which forms the enamel- 

 like surface of ' ganoid scales / but, as the term has been pub- 

 lished by me in no other way than orally in lectures, I should be 

 willing to resign it for the new dental tissue which Mr. M'Coy 

 professes to have discovered, if his claim to the discovery were 

 sound. If I mistake not, Mr. M^Coy first announced his discovery 

 in your 'August Number' of the present year, p. 124, where, 

 after animadverting on the frequent mistake of his new modifi- 

 cation of dentine for true enamel, he says : " The latter is, how- 

 ever, secreted by a distinct organ quite external to and indepen- 

 dent of the dentine, while the false enamel, which I propose to 

 call ' ganoine,' is merely produced by the calcigerous tubes of the 

 dentine becoming suddenly straighter, closer and more numerous 

 as they approach the surface " (p. 124). 



In my 'Odontography' I defined what I believe to be the 

 ' ganoine ' of Mr. M'Coy in the following words : " In some in- 

 stances, as in the teeth of the flying-fish [Exoccetus) and sucking- 

 fish {Remora), the substance of the tooth is uniform, and not 

 covered by a layer of a denser texture. In others, as the shark, 

 sphyrsena, &c., the tooth is coated with a dense, shining, enamel- 



