12 FOSSIL MAMMALIA OF THE 



The implantation of certain teeth by two or more roots in correspondingly complex 

 sockets, to which the roots do not coalesce, is a mammalian peculiarity, though not 

 common to the entire class. 



§ III. Genus — Amphitherium, Be Blainville, 1838. " Doutes sur le pretendu 

 Didelphe fossile de Stonesfield ; " ' Comptes rendus de I'Acad. des Sciences,' 

 August 20th, 1838. 



Amphigonus, Agassiz, 1835. Neues Jahrbuch fur Mineral, und Geolog. von Leonhard 

 und Bronn, Bd. iii, p. 185. (This name was proposed to express the, 

 then, ambiguous nature of the fossil.) 



Thtlacothekium, Valenciennes, 1838. Comptes rendus de I'Acad. des Sciences, 

 Septembre, 1838, p. 5/2. 



Species 1. — Amphitherium Prevostii, Owen. Plate I, figs. 21, 21a, 22, 22a, 23, 23a. 



Brit. Foss. Mammals, 8vo, 1846, p. 29. 



Thylacotherium Prevostii, Fal. Comptes rendus de I'Acad. des Sciences, Sep- 

 tember, 1838. 



PI. I, fig. 21, gives the natural size in outline, and fig. 21 a, part of the same four times 

 that size, in tint, of the original specimen examined by Cuvier in 1818, and noticed by 

 him in 1824.' The fossil partly exhibits, partly represents by impression in the matrix, 

 the left half of the lower jaw, with the fore extremity broken off. A thin layer of the 

 original bone adheres to that part of the impression which was formed by the joint or 

 ' condyle ' {b) ; the impression above gives the size and shape of the coronoid process (c) ; 

 a portion of the angle {a), remains, which is continued backward nearly as far as the 

 condyle. The part of the jaw containing the three hindmost grinders is almost entire ; 

 the outer wall of the rest of the bone is left imbedded in the Oolitic slate, but this 

 part retains seven of the molar series, with their roots, undisturbed in their sockets. 



From what has been premised of the mammalian characters of the lower jaw and teeth, 

 those which led Cuvier to conclude that the present fossil belonged to that class will be 

 readily appreciated. The convexity of the condyle and the implantation of the teeth, 

 each by two fangs, are decisive on this point. 



Ten molars are shown in the present fossil, and the two long implanted fangs are 

 exposed in seven of these teeth, lodged in deep sockets. 



A subsequently acquired half-jaw of the same species (PI. I, fig. 23, 23 a), with the whole 

 dentition of that mandibular ramus, shows that the first four teeth of the original fossil, 

 counting backward (fig. 21, a), correspond with the third, fourth, fifth, and sixth premolars, 



' "Carnassier de Stonesfield, voisin des Sarigues," Cuvier, ' Ossem. fossiles,' torn, v, pt. ii, 4to, 1824, 

 p. 349. Id., 8vo ed., 1836, torn, x, p. 484. 



