194 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 7, 



continuous tract of dentine ; they remain distinct in the middle and 

 third divisions of the crown, fig. 5 a. The fore-and-aft extent of the 

 crown is 9 lines : the depth of the jaw beneath the middle of the 

 tooth is 1 1 lines. 



Thus, of the genus Dichodon, the deciduous dentition, and the 

 permanent dentition, save as respects the last premolar, are now 

 known. 



The formula of the deciduous dentition is — 



. 3—3 I— 1 7 4—4 o 1 



«3— 3' ^T=I> ^»«i=5=32; 

 that of the permanent dentition is — 



• 3—3 1—1 4—4 3—3 . . 



The deciduous formula is the same as that of the Hog : the per- 

 manent one differs from that of the Hog only in the displacement of 

 c? 1 by the development beneath it of j? 1 , and in the functional 

 character and size of that tooth : this fact, interesting because of its 

 closer conformity to the typical diphyodont dentition, is demonstrated 

 by the difference of size of j> 1 in PI. III. fig. 4, and the first lower de- 

 ciduous molar marked ^ 1 in my former memoir, loc. cit. pi. 4. fig. 4. 



The Hog is the only existing hoofed genus that manifests, as regards 

 number, the typical dentition displayed by the Dichodon in common 

 with many other Eocene ungulate and unguiculate Mammalia. The 

 deviation in the Hog from this type is slight, being confined to the 

 non-development of ^ 1, and the early reduction of the numerical 

 formula by the loss of the small tooth, c? 1, at the beginning of the 

 molar series. 



That the Dichodon belongs to the Artiodactyle series is inferred, 

 notwithstanding the want of any direct evidence of the structure of its 

 limbs, from the more simple form and structure of^ 1, i? 2, and^ 3, 

 as compared with the true molars, and from the symmetrical rumina- 

 ting pattern of the grinding surface of the crown of the true molars. 



From the true Ruminants the Dichodon differs in the development 

 of the upper incisors and of^ 1 in both jaws, which teeth are wanting 

 in all the known existing species. 



Such feeble traces of embryotic rudiments of these teeth as have 

 been observed by Professor Goodsir and others, in the Cow and Sheep, 

 and the more conspicuous germs of upper incisors, of which one pair 

 is functionally developed, in the Camelidce^ are phsenomena that 

 derive increased significance and interest from the fact of the func- 

 tional development of the same teeth in Artiodactyle Ungulates of 

 the Eocene period. 



In the configuration of the true molars the Dichodon would seem 

 to be more nearly allied to the lluminant section of the Artiodactyla : 

 in the number and kinds of its teeth it more resembles the Hog-tribe 

 amongst the non-Ruminant section. The known facts of the deci- 

 duous dentition of the Dichodon supply an additional test of its affi- 

 nities, owing to the marked differences in the times and order of 

 succession of the permanent teeth between the Hog-tribe and the 

 Ruminants, at least of the Ox and Sheep. 



