HIPPOPOTAMUS. 13 



Pm. 4 retains in tlie main the conical form of the other premolars, but is a 

 shorter and stouter tooth, with a strongly marked cingulum. The main cone 

 has a ridged surface, and when worn the enamel may have a pentagonal outline 

 (PI. V, fig. 8). Examination of recent skulls shows the tooth to vary considerably. 

 Pm. 4 cuts the jaw considerably later than pm. 3 and is consequently less worn in 

 most skulls. 



The three molars are semiquadrate teeth with the same general type of 

 structure, and are typically bunodont and marked by a strong cingulum. In each 

 a transverse groove divides the crown into two prominent cones, each of which is 

 further subdivided by a longitudinal groove. The outline of the enamel in the 

 worn tootli is strongly lobate (PI. VI, fig. 3). Each tooth has four powerful roots 

 which diverge somewhat and hold it firmly in the jaw. M. 1 is always more worn 

 than m. 2, and m. 2 than m. 3. Unworn molar teeth that have not cut the jaw are 

 difficult to distinguish from one another. 



M. 1 has the cingulum swelling into a small tubercle on the outer surface 

 opposite the transverse groove, and a less marked swelling at a corresponding 

 position on the inner side of the tooth. 



In M. 2 the cingulum may rise into a fairly prominent tubercle on the outer side 

 of the tooth, but not on the inner side. 



M. '■) has the cingulum rising into a rather marked posterior tubercle, while 

 there may be slight swellings of the cinguluin opposite both inner and outer ends 

 of the transverse valley. 



The worn molars are well described in Tomes' ' Dental Anatomy ' l as follows : 

 "These latter, especially when worn, have a very characteristic double trefoil 

 pattern ; the four cusps in the first instance were separated by a dee]) longitudinal 

 and still deeper transverse groove ; each cusp was, moreover, trilobed. The first 

 result of wear is to bring out the appearance of four trefoils. Next, when the 

 longitudinal furrow is worn away four lobed figures result, and finally all pattern 

 becomes obliterated and a plain field of dentine surrounded by enamel alone 

 remains." 



Permanent Dentition of the Loioer Jaw (Plate V). 



The lower incisors are implanted more horizontally in the jaw than the upper, 

 and are in consequence more forwardly directed. 



]. 1 (Text-fig. 1, n) is a very large tooth with, as a rule, half its length pro- 

 jecting from the alveolus. The whole surface is coated with a thin layer of enamel. 

 Friction with i. 1 causes the upper (posterior) surface to be worn away somewhat 

 irregularly, but chiefly externally. 



1 7th ed., p. 514. 



