Ch. XX] 



MAMMALIA OF STONESFIELD OOLITE. 



407 



flatter, and devoid of all muscular depressions and ridges, one of 

 which is so prominent in the middle of this bone, represented in the 

 preceding cut (fig. 411). In saurians, on the contrary, such ridges exist 

 foii the attachment of muscles ; and to some animal of that class the 

 bone is probably referable. 



These observations are made to prepare the reader to appreciate 

 more justly the interest felt by every geologist in the discovery in the 

 Stonesfield slate of no less than ten specimens of lower jaws of mam- 

 miferous quadrupeds, belonging to four different species and to three 

 distinct genera, for which the names oiAmpldtherium, Phascolotherium, 

 and Stereognathus, have been adopted. When Cuvier was first shown 

 one of these fossils in 1818 (namely, the Amphitherium), he pro- 

 nounced it to belong to a small ferine mammal, with a jaw much re- 

 sembling that of an opossum, but differing from all known ferine 

 genera in the great number of the molar teeth, of which it had at 

 least ten in a row. Since that period a much more perfect specimen 

 of the same fossil, obtained by Dr. BucHand (see fig. 412), has been 

 examined by Professor Owen, who finds that the jaw contained on the 

 whole twelve molar teeth, with the socket of a small canine, and three 

 small incisors, which are in situ, altogether amounting to sixteen teeth 

 on each side of the lower jaw. 



The only question which could be raised respecting the nature of 

 these fossils was, whether they belonged to a mammifer, a reptile, or 

 a fish. Now on this head the osteologist observes that each of the half 

 jaws in question is composed of but one single piece, and not of two 

 or more separate bones, as in fishes and most reptiles, or of two bones 

 united by a suture, as in some few species belonging to those classes. 



Fig. 412. 



Arnphithe.rium Prevostii, Cuv. sp. Stonesfield Slate. Syn. Thylacotherium Prevostii, Valenc 

 a. Coronoid process. b. Condyle. c. Angle of jaw. d. Double-fanged molars. 



The condyle, moreover (6, fig. 412), or Fi o- 413 - 



articular surface, by which the lower jaw- 

 unites with the upper, is convex in the 

 Stonesfield specimens, and not concave as 

 in fishes^ and reptiles. The coronoid pro- AmphU herium Proderipii, Owen. 



Cess (a, fig. 412) is well developed, whereas Natural size. Stonesfield slate. 



it is wanting, or very small, in the inferior 



classes of vertebrata. Lastly, the molar teeth in the AmpJiitkerium 



