from the Shales of the Northumberland Goal-field. 363 



la dent a la loupe, on reconnatt au plus fort du renflement 

 ext^rieur une ligne circulaire qui indique la limite du ca- 

 puchon dmaill^ et de la dentine. La dentine elle-meme 



n'ofFre rien de remarquable. Les tubes calciferes 



Ceux du sommet se continuent, comme chez le Polypterus^ 

 dans I'dmail, ou ils paraissent plus roides, mais en meme 

 temps plus tins et moins rdguli^rement disposes 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' email tr^s- 

 dur, et dans lequel je n'ai pu reconnaitre ces fibres composees 

 de petits cubes superposes, telles qu'on les a reconnues chez les 

 mammif^res. L'dmail du Polypterus (fig. 12) est transparent 

 comme du cristal, sans trace de structure, et ce n'est que 

 dans sa base que p^n^trent les derni^res extremites effilees des 

 canaux calciferes de la dentine," etc. 



. Respecting Saurichthys it is stated : — " Cette difference 

 entre le socle et le sommet est encore plus frappante, lorsqu'on 

 examine leur structure au microscope ; le premier est com- 

 post de dentine, le dernier d'^mail. La cavite pulpaire est 

 un cone creux entoui-d d'un cone de dentine massive, sur 

 lequel repose le capuchon ^mailld comme dans les dents du 

 Polypt^re." 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 escaped the observation of the learned author of the 

 ' Odontography ' ? for escaped him it assuredly has, or he 



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