334 ON THE FOSSIL^BONES OF PACHYUEUaiATOUS QUADIIUPEDS. 



the same time these two pieces show us at once that the molares with 

 eight denticuli do not belong to the lower jaw, otherwise they would 

 be placed between the two just mentioned, Avhich is not the case. 



Hence we must conclude^ that they belong to the opposite jaw. 

 This, in fact, is proved by a fragment of a jaw, also presented by Mr. 

 Jefferson, and which we give, plate 20, fig. 4. "We may there see a 

 molar^with eight denticuli in its place, similar to those I have 

 described in a detached state. 



Moreover," it is positively ascertained by the fi-agments of M. 

 Michaelis, plate 20, figs. 2, 3, and 5, as well as by the skull of Mr. 

 Peale's skeleton, that in front of the tooth with eight denticuli of the 

 upper jaw, there is one with six points, almost similar in every respect 

 to that of the lower jaw. 



The disposition of the jaw teeth of the adult is this, that it has four 

 belonging to each jaw, that is, two with six denticuli, and two with 

 eight denticuli above ; two with six denticuli, and two with ten 

 denticuli below. 



The back molares, whether of eight or of ten denticuli, have besides 

 a small fang, more or less irregular, which has so far deceived some 

 engravers, as to lead them to represent it as if forming an additional 

 pair of denticuli. Others, on the contrary, have sometimes given the 

 last pair of real denticuli, when a little smaller than usual, as if it 

 were nothing more than a little fang*. These inaccuracies of certain 

 figures led me to believe at the period of my first edition, that there 

 might be jaw-teeth with eight denticuli below, between those of six 

 and those of ten denticuli, but observation has undeceived me. 



From this difference of the back molares upper and lower, we may 

 observe that they bear the same relative proportions to each other in 

 the mastodon that they do in the hippopotamus, which has the last 

 lower molar a little more complicated, and with a denticuli more than 

 the last of the upper jaw. 



But besides these eight molares which remain in the adult, there 

 are others placed before these in young subjects, which are cast in 

 succession. 



The young jaw presented to our Museum by Mr. Jefferson, and 

 represented plate 21, figs. 3 and 4, affords a proof of this : we may 

 there see, quite in the rear, the remains of a large cell which ought to 

 contain the germ of a large molar, of the number of whose denticuli 

 we are ignorant, as it has been lost, but which was probably the molar 

 with ten denticuli, which we may observe in the adult jaws. 



In the front of this cell is a tooth, which came out of the socket, but 

 not out of the gum, as its knobs are as yet perfectly unimpaired : there 

 are six pairs of them. There is moreover in front a molar of six points, 

 but now somewhat worn, and in the most forward part of the jaw the 

 remains of an alveolus are to be seen, indicating that a tooth had been 

 there. 



* Thus, in the engraving of the lower jaw in the British Museum (Philosophical 

 Transact, p. 34), the fifth pair of denticuli are not sufficiently marked ; on the other 

 hand, the fang of the upper jaw of Peale's skeleton, given by Bonn, if in reality it 

 be not a suppositious tooth which they have fastened to the skull, is much too 

 strongly marked. 



