1857.] OWEN — PLIOLOPHUS VULPICEPS. 59 



distinctly marked by the modifications of the lower jaw, especially by 

 the relative dimensions of the parts of the ascending ramus which 

 give the extent of attachment of the biting (temporal, i) and grind- 

 ing (masseteric and pterygoid, 29) muscles respectively. In the 

 shape of the mandible Pliolophus most resembles Tapirus among 

 existing mammals, and the Palceotherium among the extinct ones 

 in which that shape is known. Unfortunately no mandible of a true 

 Lophiodon has yet been found so entire. 



As far as the portion of the skull of the Hyracotherium leporinum 

 permits the comparison to be made, there is a close general resem- 

 blance between it and Pliolophus ; but the skull of the Hyracothere 

 is broader, at the orbital region, in proportion to the length of the 

 antorbital or facial part*. The orbits are both absolutely and rela- 

 tively larger ; they are also rounder and have a lower position. The 

 straight upper contour of so much of the skull of the Hyracothere 

 as has been preserved, the size and position of the antorbital fora- 

 men, the course of the maxillo-premaxillary suture, and the forma- 

 tion of the bony nostril by the nasals and premaxillaries exclusively, 

 are further indications of the affinity of Hyracotherium to Plio- 

 lophus. 



This affinity is decisively shown by the more important characters 

 derived from the dentition. 



Description of the teeth of Pliolophus : Plate III. — As in the 

 Hyracotherium, and, indeed, as in almost every species of Eocene qua- 

 druped yet discovered, the Pliolophus presents the type-dentition 

 of the placental Diphyodont series, viz. : — 



• 3—3 1—1 4—4 3—3 . . 

 1 §-5> c i-i>i>i-i'™3=§ -44. 



The incisors, PL II., i 1, 2, 3, are preserved in the lower jaw with 

 marks of attrition on their crowns demonstrating corresponding 

 teeth of the same number, 6, and of similar size, in the upper jaw, 

 from which the alveolar part of the premaxillaries had been broken 

 away. 



The lower incisors, PL II. fig. 1, u, 2, 3, form a semicircle termi- 

 nating the slender sloping symphysis mandibulae, s, and projecting 

 parallel with it, so as to be almost procumbent, PL II. figs. 3 and 4, i. 

 Their crowns present the common wedge-shaped form, with the 

 trenchant border obliquely beveled off, and the more so from the 

 first, i 1, to the third, i 3 : they slightly decrease in size, or at least 

 in length of crown in the same course ; but the outer incisor, i 3, is 

 not so small relatively as in the Tapir. The degree of attrition to 

 which they have been subject produces a certain breadth of the 

 trenchant border. 



The canines, ib. c, are small in both jaws : only the crown of the 

 right lower one, PL II. fig. 3, c, is entire : it is about 4 lines long, 

 in the form of a slender cone, inclined a little forwards, with the 

 front border convex, the hind one more nearly straight. The lower 

 canine is separated by an interval equal to its own basal breadth, viz. 

 about 2 lines, from the outer incisor, and by an interval of 5 lines 



* Op. cit. pi. 21. fig. 2. 



