VOL. LXXVI.] PHILOSOPHICAL TRANSACTIOKS. 155 



first are all shed at a certain period of life, and in the mean time new ones are 

 formed above, under, or at the sides of the primordial or temporary teeth, but 

 in different sockets. The grinders are not all renewed, but in general 3 when 

 there are 6, and 2 when there are 5. Nature however is not always uniform in 

 this operation. Mr. John Hunter has given a very interesting and complete 

 natural history of the teeth, in which these observations are stated. In the 

 crocodile the succeeding or secondary teeth appear even when the animal's head 

 is equal to 2 feet ; that is, when it has acquired -f of its usual growth. When 

 they grow too fast, before the temporary tooth is shed, they perforate the side 

 of the bone, at the part where they meet with the least resistance. Instances 

 of this variety occur in the large crocodile's head in my collection. 



In all quadrupeds, the enamel is, of the solid part of the teeth, the first 

 formed, making a cavity, in which the other bony substance is deposited, and 

 formed by lamellae placed one within another, as is observed by Mr. John 

 Hunter, in the work already mentioned, p. 92. To this the root is added, 

 which is filled in the same manner till the tooth is long enough to pierce 

 through the gums. But in the fossil jaw-bones of St. Peter's mountain, a small 

 secondary tooth is formed, with its enamel and solid root at once, within the 

 bony substance of the primordial or temporary tooth itself, as is to be seen in 

 the small fragment now in the British Museum, and in fig. 8, abcde ; which, 

 by continuing to grow, seem to make by degrees sufficient cavities in the bony 

 roots of the primary teeth : but what becomes of them at last, and how they 

 are shed, I am not able to guess. I have one in my collection, where the suc- 

 ceeding tooth is entirely formed within the centre and substance of the primor- 

 dial tooth. In fig. 6, a little oval cavity is observable, which has been the seat 

 of a new or secondary tooth. 



§ 6. The maxilla inferior of the incognitum, sent by me to the British 

 Museum, is a most magnificent specimen, having 14 teeth. A similar one, 

 somewhat longer, as it measures 3-*- feet, in my own collection, has also 14. 

 Another fragment of the left side, 2 feet long, and 8 inches broad, shows the 

 primordial and succeeding teeth in the clearest manner. The specimen, of 

 which I sent a drawing, fig. 8, to the illustrious President of our Society, Sir 

 Joseph Banks, is still more useful to confirm the mode of dentition than any 

 other I have in my Museum. 



^ 7. Several ribs and the phalanges of the toes of the fore- feet, a specimen of 

 which I sent in a fragment from the same rock, of about a foot long and 8 

 inches broad, may serve as another proof of the difference between these and the 

 crocodile's toes, when compared with the still valuable, though neglected, ske- 

 leton in the British Museum ; which I am sorry I could not make a drawing of, 

 having been too much employed on other objects. All these characteristic 



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