336 ox THE FOSSIL BONICS OF PACHYDICUJMATOUS QCADRUPKDS. 



This figure, v»itli six lozenges, is the result of an incomplete detri- 

 tion ; when detrition is not so far advanced, there are only slight trun- 

 cations ; and when the tooth has not as yet protruded from the gum, 

 the crown merely presents three-notched knobs, somewhat depressed 

 in their centre. 



The generosity of Mr. Jeflferson has put me in possession of two of 

 these germs, which have not undergone the slightest alteration. One 

 of them is still adhering to the jaw, (see plate 21, figs. 3 and 4). 

 That which is detached is 0,1 long, and 0,065 broad. 



On the other hand, when the process of detrition is farther advanced, 

 as is the case with the tooth of Pallas (Act. Petrop. 1777, second 

 part, plate 9, fig. 4), the lozenges become united in pairs, in trans- 

 verse bands. 



At length detrition proceeds so far as to unite these bands together, 

 thereby forming a single irregular disc of osseous matter, as is repre- 

 sented by the tooth of plate 22, fig. 4, which is that found by M. Legris 

 of Bellisle. It is 0, II in length, and 0,08 in breadth. 



The teeth of six denticuli, or of six lozenges, vary in length from 

 0,095 to 0,12, and the longest are not always the broadest, so that 

 there are many of them more or less square. 



A difference quite as essential consists in this, that the transverse 

 hills are more oblique in those that are narrower in proportion, and 

 more at right angles in those that are square. 



The specimens which I have had an opportunity of observing in 

 their places in the lower jaws being all of the more oblique species, I 

 have reason to believe that it is the upper teeth that are more square 

 and that have their hills more transverse. This is the more probable, 

 as the same dilFerence in the direction of the hills is observable with 

 regard to the back molares. 



The two teeth contributed bv Longueil at a very early period, 

 one of which is represented by BufFon, in his supplement, vol, v, plate 5, 

 were square in this way, and very transverse. One of them mea- 

 sures 0,11 by 0,085. 



The teeth with six oblique denticuli differ from each other in 

 relative length and breadth. The young jaw (plate 21, figs. 3 and 4), 

 demonstrates that the smaller, which are also the narrower, are 

 l)laced before the others. In fact, the posterior tooth there, measures 

 0,095 by 0,066, and the anterior 0,08 by 0,055. 



Let us now proceed to the teeth that have ten denticuli, and are con- 

 tracted behind, that is to say, the lower back molares.. They vary in 

 the degrees of detrition, as much as those of six denticuli. 



Their size is certainly calculated to excite surprise. That in the 

 Museum of Ebel, described by the Baron de Bock, and which is half- 

 worn down, is 0,237 in length. 



We have one in the King's Museum, contributed by the Duke of 

 Placenza, the points of which are quite unimpaired ; it is 0,230 in 

 length, and 0,11 in breadth. I have given an engraving of it (plate 19, 

 fig. 2). 



Another equally unimpaired, contributed by M. Dufrien, measures 

 0,225 by 0,1. 



That which is represented in figs. 1 and 3, and which is now some- 



