10 PLEISTOCENE MAMMALIA. 



The Hippopotamidge differ from the other families in the broad and rounded 

 character of the muzzle, in the larger size and persisted pulps of the canines and 

 incisors, and in the Pact that the digits are subequal, all reaching the ground in 

 walking and being terminated by short rounded hoofs. 



a. The Skull (Plates I— IV). 



The brain cavity is very small, and the cranium little raised, so that a line 

 drawn from the occipital crest to the nose is nearly- parallel to the palate. The 

 occipital crest is strongly developed, while there is a less marked sagittal crest 

 which bifurcates in front to form two divergent ridges extending towards the 

 orbits. The orbits are extremely prominent, particularly in old males, and 

 project outwards and upwards. Postorbital processes from the frontals and 

 jugals almost or quite meet and complete the postorbital bar. The lachrymal is 

 peculiar; it occupies a relatively large space on the roof of the skull, while within 

 the orbit it is dilated into a delicate thin-walled capsule. The lachrymal canal is 

 deep within the orbit. The face contracts in front of the orbits and then expands 

 into an enormous muzzle which lodges the husre incisor and canine teeth. The 

 nasals, which are very lone;, are much widened at the posterior end, while 

 anteriorly they diverge. The suborbital foramen is large. The palate is long 

 and narrow, and the palatal aspect of the cranium is remarkable for the fact 

 that the two series of teeth, instead of converging when followed forward, run 

 parallel to one another or even diverge slightly. The hamular process of the 

 pterygoid is frequently but not invariably prominent. There is a wide glenoid 

 cavity formed by the squamosal, but the post-glenoid process is not very 

 prominent. There is a very distinct paroccipital process of the exoccipital, but^it 

 is not nearly so prominent as in the pig. There is a long, narrow, auditory 

 meatus terminating in an inconspicuous foramen. The mandible is immensely 

 large and heavy with a long symphysis. It expands in front to lodge the great 

 canine teeth, and in consequence is wider at the anterior than at the posterior end 

 of the symphysis. The coronoid process is very small and somewhat recurved, 

 while the angle is greatly expanded and bent outwards, ending in a downwardly 

 and forwardly projecting process. 



e. The Dentition (Plates V ami VI). 



The incisor and canine teeth of Hippopotamus amphibius are very remarkable, 

 being immensely large and provided with persistent pulps. The grinding teeth on 

 the other hand have a primitive bunodont character. The dental formula is i. | 

 c. j pm. f m. f. 



The homology of the incisor teeth has been a matter of considerable difference 



