From the Shales of the Northumberland Ooal-field. 363 
la dent 4 la loupe, on reconnait au plus fort du renflement 
extérieur une ligne circulaire qui indique la limite du ca- 
puchon émaillé et de la dentine. La dentine elle-méme 
n offre rien de remarquable. Les tubes calciféres ....... 
Ceux du sommet se continuent, comme chez le Polypterus, 
dans l’émail, ot ils paraissent plus roides, mais en méme 
temps plus fins et moins régulitrement disposés que dans la 
dentine.” 
Of Polypterus the same author writes as follows :—“ Cette 
dentine forme la plus grande partie de la dent; elle n’est re- 
couverte qu’au sommet par un petit capuchon d’émail tres- 
dur, et dans lequel je n’ai pu reconnaitre ces fibres composées 
de petits cubes superposés, telles qu’on les a reconnues chez les 
mammifeéres. L’émail du Polypterus (fig. 12) est transparent 
comme du cristal, sans trace de structure, et ce n’est que 
dans sa base que pérétrent les derniéres extrémités effilées des 
canaux calciféres de la dentine,” ete. 
_ Respecting Saurichthys it is stated: —‘‘ Cette différence 
entre le socle et le sommet est encore plus frappante, lorsqu’on 
examine leur structure au microscope; le premier est com- 
posé de dentine, le dernier d’émail. La cavité pulpaire est 
un cdne creux entouré d’un céne de dentine massive, sur 
lequel repose le capuchon émaillé comme dans les dents du 
Polyptore.” This description of the structure of the tooth of 
Saurichthys is very different from that given in the ‘ Odonto- 
graphy’ (page 170), where the cap of enamel is certainly de- 
scribed, but not recognized as such, the author apparently not 
being aware of the difference between the base and the summit, 
pointed out by M. Agassiz. And indeed the description seems 
to be confined to the enamelled or upper portion alone, the 
basal portion evidently having been deficient in the specimen 
examined. 
Similar passages might be quoted respecting Lepidosteus ; 
but perhaps enough has been said on the supposed recent dis- 
covery of the ‘‘ enamel-tipped spear teeth.” We have seen 
that M. Agassiz fully described and accurately figured this 
form of tooth in four genera (Pl. XVI. figs. 3,4) between twenty 
and thirty years ago (1833-1844); and we have determined 
its existence in four other genera, and have likewise verified 
the accuracy of M. Agassiz’s observations in Pygopterus, 
Lepidosteus, and Saurichthys, making in all eight in which 
a cap of enamel is found. It is therefore highly probable 
that, when the subject is fully investigated, enamel-tipped 
teeth may prove to be not at all uncommon. But how has 
all this secant the observation of the learned author of the 
‘Odontography’? for escaped him it assuredly has, or he 
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