532 Geological Society. 



short transverse bar — the rudiment of the middle tubercle in the 

 preceding tooth. In Coryphodon the sinking of the middle of each 

 transverse ridge seems a step towards the more complete bifurcation 

 of the ridge in Pliolophus. 



The mandible and lower molar teeth of the Hyracotherium are 

 unknown ; but, in regard to the upper molars, Pliolophus approaches 

 Hyracotherium in the characters by which it deviates from other 

 Lophiodonts. 



The differences in the dentition, and in the part of the skull of 

 the Hyracotherium which can be compared, between that genus and 

 Pliolophus were pointed out in detail ; but the degree of resemblance 

 is such as to lead the author to adopt the idea, first broached by the 

 late Mr. W. H. Turner*, that Hyracotherium is more nearly or 

 essentially allied to Lophiodon than to Chceropotamus. The third 

 trochanter on the femur of Pliolophus, and the association of three 

 metatarsals in one portion of the matrix, as if belonging to the same 

 hind foot, confirm the essentially perissodactyle affinities of that genus 

 as shown by the skull and teeth. Pliolophus and Hyracotherium 

 form, in the author's opinion, a well-marked section in the Lophiodont 

 family, which seems to have preceded the Palseotherian family in the 

 order of appearance, and to have retained more of the general ungulate 

 type than that family. This is shown by the graduation of the tapiroid 

 modification of the molar teeth into one more nearly resembling that 

 of the Anthracotheria and Chceropotami ; by the absence of the 

 postero-internal cone on the ultimate premolar, by which all the 

 premolars are, as in Artiodactyles, less complex than the true molars ; 

 by the form and position of the nasal bones and the structure of the 

 external nostril. 



In regard to the evidence of closer adherence to type shown by 

 the dentition of Pliolophus and other ancient mammals, the author 

 concluded by remarking that the dental formula of the Oolitic ge- 

 nera Thylacotherium, Spalacotherium, and Tricon^ ion accords, by the 

 unusual number of small and similarly shaped molars, with a less 

 specialized type than that of the Diphyodont Mammalia, which he 

 terms the more general vertebrate type, and exemplifies that which is 

 shown by Reptiles, Fishes, Cetacea, and certain Armadillos : if the 

 Plagiaulax of the Purbeck beds departs from this type in the reduction 

 of its true molars to two, it singularly manifests its closer adherence 

 to the type-dentition of its order by having its peculiarly shaped 

 premolars in the typical number three ; whereas the only existing 

 marsupial genus with premolars of such a shape, viz. Hypsiprymtms, 

 Illig., has those teeth reduced to owe in each molar series. 



The exception offered by the Plagiaulax is like that of Proteles 

 amongst the Canidce, in which wild species of Dog the true molars 

 are reduced to one in each series. But this exception does not 

 invalidate the generalization from the dentition in the rest of the 

 Dog-and-Wolf family, any more than Plagiaulax affects the general 

 expression of the facts presented by the dentition of the great ma- 

 jority of the known eocene Mammalia, of which the author in 

 conclusion cited thirty-seven genera which exhibited the typical 



* Annals and Magazine of Natiual History, Dec. 1850, p. 397. 



