xi. MAMMALIA OF STONESFIELD. 233 



never exhibit,, as in these fossils, true and false molars with 

 different crowns. 



The lower jaws of reptiles generally are composite ; and Dr. 

 Grant regarded the Stonesfield jaw in the same light, in con- 

 sequence of a small groove at the base, which he supposed 

 to separate the dentary and opercular pieces ; but there is really 

 * no deep dividing fissure between dentary and opercular,' but 

 a distinct smooth groove, as in myrmecobius and wombat, the 

 mark of a blood-vessel. 



It is needless to pursue this argument; the result in favour of 

 the mammalian nature of the jaw is now universally accepted. Three 

 genera have been recognized at Stonesfield, and have received the 

 names of Amphitherium, Phascolotherium, and Stereognathus. 



Diagram LXXVII, Amphitherium Prevostii. Oxford Museum. Natural size. 



Amphitherium Prevostii (Diagram LXXVII.). A small ferine 

 animal, with a lower jaw filled with minute teeth, sixteen on each 

 side; viz. six molar, six premolar, one canine, and three incisors. 

 The molars have several cusps, three being principal ; the premolars 

 have one principal cusp. Each has a double fang, rather deeply 

 rooted in the compressed jaw. The diagram referred to represents 

 the specimen which was obtained by Buckland, and submitted to 

 the examination of Cuvier in 1818. It is the left ramus, seen 

 internally. The fangs of the anterior molars are exposed; the 

 crowns of all. The coronoid process rises boldly in a broad thin 

 expansion, having below it a strong convex, prominent, articular 

 condyle, and an equally conspicuous and angular element. There 

 is a little horizontal groove below the last molars mark of a blood- 

 vessel. 



Diagram LXXVIII. represents a second specimen placed by 



Diagram LXXVIII. Amphitherium Prevostii. OxforcLMuseum. Natural size. 



