66 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 20, 



fication in the grinding surface of its upper molars, to the same 

 secondary group of Ungulata as includes the Chceropotamus. 



Most comparative anatomists who have studied the unique evi- 

 dences of that old eocene genus, since my description of them, have 

 arrived at and have recorded the same conclusions*. The only 

 dissentient from them was Mr. H. N. Turner, jun., a most promi- 

 sing and acute naturalist and anatomist, who died too soon for the 

 interests of science. In a very able and characteristic paper " On 

 the Evidences of Affinity afforded by the Skull in the Ungulate 

 Mammalia f," Mr. Turner points out the basal expansion of the 

 nasal bones, the absence of the superorbital foramen and groove, 

 and the slightly marked depression for the origin of the obliquus 

 inferior oculi, within the orbit, as indications of the perissodactyle 

 affinities of the Hyracotherium : he, also, most acutely discerned in 

 the rudimentary oblique ridges upon which the small intermediate 

 tubercles were developed in the molar teeth rudimental homologues 

 " of the bent transverse ridges in the Rhinoceros, Tapirus, Palceo- 

 therium, and other allied genera ; " but the degree of resemblance 

 of the molars to those of the Anthracotlieria and Choeropotami was 

 such as led Mr. Turner, in regard to the question of the artiodactyle 

 or perissodactyle affinities of the Hyracotherium, to admit, "to 

 whichever group, then, this little animal be referred, the teeth will 

 present marked exceptional characters, and, therefore, it becomes 

 more necessary to seek for further evidence J." 



This evidence I believe to be now afforded by Pliolophus, on the 

 ground of the following illustrations of its close affinity to Hyraco- 

 therium. 



Like that genus, its upper true molars exhibit the modification of 

 the Lophiodont type of dentition in the more circumscribed and 

 better-developed enlargement of the middle of the connecting oblique 

 ridges : it also shows the more simple structure of the last premolar 

 by the same non-development of the postero-internal cone. Hyra- 

 cotherium differs from Pliolophus in the more distinct development 

 of the intermediate tubercles, especially of the second or posterior 

 one in p 4 : the cingulum girts the crown uninterruptedly in the 

 true molars and last two premolars. Hyracotherium differs, also, 

 in the wider interval between the first and second premolar. 



It may be questioned whether these differences are of generic 

 importance, or whether, with those before pointed out in the con- 

 figuration of the skull, they may not merely indicate another species 

 of this genus which seems to be peculiar to our London Clay. In 

 resolving this question I have been influenced by comparing the 



* De Blainville, Osteograpbie des Anthracotheriums et Chceropotames, fasc. 

 xxi. p. 194. Hyracotherium described and figured " d'apres un platre assez bon, 

 envoye a la collection du Museum, par M. R. Owen." 



Gervais, Summary of Ungulata observed in France : — 



11. Chceropotamina. Entelodon, Chceropotamus, Hyracotherium. 

 Pal. Franc, descr. of pi. 36. p. 6. 



t Ann. and Mag. Nat. Hist. Dec. 1850, 2nd ser. vol. vi. p. 397. 



J Loc. cii. p. 408. 



