260 MAMMALIA. 



m. 3 or 4. The lower incisors are small and uniform, the crown pointed, 

 a little recurved, and with a small posterior heel. The lower canine may 

 have had a bifid root, but this is uncertain. The premolars and molars in 

 both jaws exhibit a bifid root and a conspicuous cingulum, while the crown 

 bears three cusps in one antero-posterior series, the three nearly equal in 

 the true molars, the lateral ones much reduced in the premolars. One 

 mandibular ramus, originally named Triacanthodon, shows the fourth 

 deciduous tooth shaped like the true molars, with the fourth premolar 

 beneath it ready to take its place. The coronoid process is large, and the 

 articular condyle is not above the level of the dentition ; the symphysis 

 rises rather abruptly from the lower border, and there is a slight inflection 

 of this border near the angle of the mandible. This is the commonest 

 mammalian fossil in the Middle Purbeck Beds of Dorsetshire, and the 

 mandible of the typical species, Triconodon mordax, measures 0'035 m. in 

 length. The mandible named T. major would probably be as much as 

 0'06 m. in length. Very similar jaws occur in the Upper Jurassic of 

 Wyoming (Priacodon ferox, fig. 152c). 



Spalacotherium. Definitely known only by slender mandibular rami 

 from the Middle Purbeck Beds of Dorsetshire. Dental formula : i. 3, 

 c. 1, pm. 4, m. 6. The molars are tritubercular, there being one relatively 

 large outer cusp, a pair of smaller inner cusps, and an inner cingulum. 

 The coronoid process is very large, and the articular condyle of the 

 mandible is placed well above the level of the dentition. The mylohyoid 

 groove is conspicuous. The typical species is S. tricuspidens, with mandible 

 about 0'03 m. in length. Menacodon is a related genus from the Upper 

 Jurassic of Wyoming. 



Before the close of the Cretaceous period, the typical 

 American family of polyprotodont marsupials known as opos- 

 sums or Didelphyidae, appear to have established themselves 

 at least in the northern half of the continent, for detached teeth 

 almost indistinguishable from those of the existing Didelphys 

 occur in the Laramie Formation of Wyoming (Didelphops). It 

 is still more interesting to note, however, that the same animals 

 also existed in Europe in considerable numbers during the early 

 part of the Tertiary period, and did not become extinct here 

 until the Miocene. The European jaws bear the generic name 

 of Peratherium (or Oxygomphius), but they merely differ from 

 those of Didelphys in the relative proportions of certain teeth, 

 and are thus scarcely distinct. They occur in the Lower 

 Eocene of Essex and in the Upper Eocene of Hampshire ; in 

 the Upper Eocene of Debruge and Quercy, in France ; also in 



