MARSUPIALIA. 257 



have been discovered; Cimolestes and other names have been 

 given to detached teeth from the Laramie Formation. 



Pliascolotherium (fig. 150). Known only by three imperfect man- 

 diliular rami of P. bucklandi from the Stonesfield Slate, each exposed from 

 the inner side. Mandibular dental formula: i. 4, c. 1, pm. 2, m. 5. The 

 incisors are slender and styliform, and the canine is nearly similar, though 

 larger. Behind the canine is a short diasterna, and then follow seven 

 nearly similar teeth in continuous series, each with two roots. All these 

 teeth exhibit a conspicuous basal ridge or cingulum, and the crown 

 comprises one large central cone with a smaller cusp in front and behind. 

 The coronoid process is enormous, and the articular condyle, on about the 

 same level as the dentition, is borne by a slender pedicle. The lower 

 border of the jaw towards the angle seems to have been much inflected. 

 The type mandible measures only 0'035 m. in length. 



Amphitherium. Four imperfect mandibular rami from the Stonesfield 

 Slate are referred to this genus, three to the typical species A. prevosti, 

 the fourth to a distinct species A. oweni. The largest measures only 

 0-025 m. in length. Though a little uncertain, the mandibular dental 

 formula seems to have been thus : i. 4, c. 1, pm. 5, m. 6. The incisors 

 are small and styliform, and the canine appears to have had a double 

 root. The molars are remarkably complex, of a tritubercular-sectorial 

 type. The coronoid process is large, and the articular condyle is supported 

 on a pedicle well above the level of the dentition. The angle of the man- 

 dible is slightly inflected. The mylohyoid groove is conspicuous. 



FIG. 161. 



Triconodon mordax; right mandibular ramus, outer aspect, nat. size. U. 

 Jurassic (M. Purbeck Beds); Swanage, Dorsetshire. (After E. Willett.) 



Triconodon (rig. 151). The best known of the Mesozoic Polyprotodonts, 

 both the upper and lower dentition having been discovered. It is remark- 

 able for the reduction in number of the molar teeth, and the simple 

 character of the premolars as compared with the molars. The upper 

 incisors are unknown, but there is a large upper canine with bifid root, 

 and this is directly followed by four premolars and three or four true 

 molars. The mandibular dental formula is as follows : i. 3, c. 1, pm. 4, 



w. 17 



