118 MR. H. G. F. SPURRELIi ON THE [Feb. 6, 



have to act as grindstones. The comparative roundness of the 

 condyle, the large extent of the articular surface over which it 

 can play, and the presence of an emenentia articularis on to 

 which it can glide, all contribute to increasing the range of these 

 movements. The structures, moreover, are capable of considerable 

 modification to meet the various requirements of ruminants, 

 rodents, primates, &c. 



The mouth in Type 1 has to allow its possessor to seize and 

 hold other animals, and for this reason the wide gape which the 

 type allows is required. First the animal must bring its long 

 canine teeth to bear on its prey ; then it must be able to bring its 

 hindmost molars direcbly to bear on the larger bones and the flesh 

 of the prey. Therefore, in animals of this type, the corners of the 

 mouth extend far back. 



Text-fi^. 41. 



Showing len^liening of the jaws forward in order that the incisors may be widely 

 separated whilst the molars are slightly separated and not thrown much out of 

 the parallel. 



The mouth in Type 2 has to allow its possessor to crop grass 

 or gnaw oflf pieces of roots, &c. These morsels of food are then 

 ground up by the molars. For neither of these purposes is a very 

 wide gape required. A very slight separation of the molars is 

 necessary ; but in some animals, particularly the rodents, a rather 

 wider separation of the incisors is required. As the progressive 

 widening from back to fi-ont of the gap between the teeth is small 

 when jaws of type 2 are opened, the jaws are often lengthened 

 forwards. This allows the same movement which sepai'ates the 

 molars at the back slightly to separate the incisors in front more 

 widely (text-fig. 41). As it is only the incisors, not the molars, 

 which are required to break up lai-ge pieces of food outside the 

 mouth, the oral fissure is small. Another reason for this is that the 

 molars require muscular cheeks to help the tongue in placing the 

 food between them. 



It is perhaps the difticulty of striking a balance between the 

 pi-oper separation of the molars and incisors which keeps the angle 

 E A C (text-fig. 41) greater than a right angle in animals with 

 incisors. In animals like the Elephant and the Manatee, in which 



