52 THE NOTOG^IC REALM. [CHAP. 



molar teeth consist of three simple compressed trenchant cusps 

 arranged in a longitudinal line ; the upper ones biting on the 

 outer side of those of the lower jaw. In the upper jaw the 

 number of teeth is still unknown, but the lower jaw carries three 

 pairs of incisors, four of premolars, and either three or four of molars, 

 in addition to the tusks or canines, which are implanted by two 

 distinct roots. In this respect the latter teeth present an approxi- 



FlG. 6. INNER SURFACE OF RIGHT HALF OF LOWER JAW OF TriCOHodon, 

 OR ALLIED FORM. 



mation to those of the bandicoots, where the root of the canine is 

 partially divided by a longitudinal groove. A second family 

 (Spalacotheriidce), likewise represented in the upper Jurassic rocks 

 both of Europe and the United States, is distinguished by the 

 cusps of the molars being arranged in a triangle, with the apex 

 pointing inwards in the upper, and outwards in the lower jaw ; 

 these teeth being similar in structure to those of the marsupial 

 mole. 



Of more interest is the large family of the Amphitheriida, 

 which may be taken to include a great number of forms apparently 

 agreeing with the opossums in having four pairs of lower incisor 

 teeth. The lower molars never consist solely of three simple 

 cusps arranged in a straight line like those of the Triconodontida, 

 or in a triangle like those of the Spalacotheriidce. Among these 

 forms the genus Phascolotherium, from the lower Jurassic Stones- 

 field Slate of Oxfordshire, appears to have had only seven pairs of 

 cheek-teeth ; the lower molars having three cusps arranged in a 

 longitudinal line, of which the middle one is considerably larger 



