548 ORIGIN OF MAMMALS xix. 7- 



vegetable matter. The cheek teeth carried longitudinally arranged 

 rows of cusps, presumably used for grinding the food. The arrange- 

 ment of the muscles can be deduced from the jaws. The temporal 

 muscle was small and there was a small coronoid process on the man- 

 dible for its attachment. There was therefore no wide sweeping up and 

 down movement of the jaw as in carnivores. On the other hand, there 

 was a large masseter, pulling anteriorly, and pterygoid transversely 

 and a shallow glenoid fossa. All these features will be found again in 

 placental herbivores. The relationship of these animals to earlier and 

 later types is quite uncertain. It has been suggested that the multi- 

 tuberculates gave rise to the monotremes or marsupials, but the 

 features they have in common with these are mostly those of all 

 primitive mammals. 



The symmetrodonts and eupantotheres are imperfectly known but 

 probably included various carnivorous and omnivorous types, not 

 unlike modern opossums and some insectivores. The lower jaw 

 was formed of a single dentary bone, the hinder jaw bones having 

 presumably already formed ear ossicles. In * Amphitherium there were 

 4 pairs of incisors on each side of the lower jaw, one canine and 11 

 cheek teeth, 4 of these being preceded by milk teeth and hence classed 

 as premolars. Several different sorts of eupantotherian are known from 

 the Jurassic and could have given rise to the earliest placental insec- 

 tivores, which appear in the late Cretaceous (p. 583). The reason for 

 believing in this relationship is the cusp-pattern of the teeth. 



8. Original cusp-pattern of teeth of mammals 



In the symmetrodonts and pantotheres the teeth were so arranged 

 as to bite against each other. The lower cusps formed a triangle, with 

 a surface behind, the talonid or heel, with which the main cusp of the 

 upper molar made contact (Fig. 321). The upper cusps also formed 

 approximately a triangle and this 'tribosphenic' condition of the 

 molars is believed to have been the plan from which the modern 

 mammalian condition is derived. The apical cusp, which lies on the 

 inner side of the upper molars and the outer side of the lower molars, 

 was at one time believed to represent the original reptilian cone and 

 was therefore called the protocone in the upper and protoconid in the 

 lower molars. The other two cusps are called the paracone (paraconid) 

 in front and metacone (metaconid) behind (Fig. 321). The separation 

 between these latter cones is not sharp in the pantotheres, especially 

 in the upper jaw, and we shall find this condition again in the earliest 

 placental mammals. 



