288 PROCEEDINGS OF THE ACADEMY OF [1887. 



Amphitheri'um, on the other hand is allied by the structure of the 



Fig. 2. Portion of the right mandi'ole of Amphilherium. 



mandible and separated by the pattern of the molars, which lack the 

 third cusp. The last two genera are somewhat isolated. 



(la) An allied family is the Phascolotheridae^ embracing Phas- 

 colother'mm and Tlnodon, in which there is a diastema behind the 

 canine ; true premolars are few or wanting', and the mandibular 

 angle is represented merely by the inflection of the lower border. 

 The molars are like those of the Triconodontidae except that the 

 anterior and posterior cusps are smaller and are slightly rotated in- 

 wards. 



(2) Spalacotheridae^ . Tliis family embraces Spalacotherium 

 and Menacodon. Tlie mandible is without a distinct angle, like that 

 in Phascolotherium, but shallower and with a more elevated condyle. 

 There is no diastema. The incisors and canines are somewhat similar 

 to those of Trlconodou. The premolars have strong anterior and 

 posterior basal cusps. The anterior and posterior cusps of the 

 molars are strongly rotated inwards. 



Omnivorous Sub-Group. 

 B. The families embraced in this division are quite closely 

 inter-related both as regards their dentition and diet. The 

 genera vary from middle to large size. The typical forms (Perales- 

 tidae) were probably omnivorous. The incisors are not known, the 

 canines are large and erect, the premolars have prominent basal 

 cusps. Instead of an internal cingulum, the lower molars have a 

 more or less prominent internal row of low cusps. The condyle is 

 usually on or below the molar level. 



2. This name is preferable to Tinoiiontidne recently proposed by Professor 

 Marsh, since Prof. Owen's genus is much the best known. It is quite probable 

 that the Phascolotheridae will be subsequently united with the Tticotiodontidae, 

 linked by intermediate forms with premolars. 



3. The post-canine teetli of Tinodon have not as yet been fully described. 

 ]. This family name was proposed by Professor Marsh, loc. cit. p. 340. 



