10 



THE GEOLOGIST. 



the supposition that any degree of wear could have worn the teeth of 

 Didymodon down to the condition of Heterohyus. 



In Dichodon the well-marked bicrescentic form of the molars, and 

 the absence of the tendency to develope 4 pyramidal cusps, with traces 

 of a posterior talon exhibited in Didymodon, render further com- 

 parison unnecessary. 



In Aplielotherium Duvernoyi, Gerv., derived from the Paris gypsum, 

 which bears many curious points of resemblance to Didymodon, and 

 which I only know by Gervais' figures, the view of the 1st and 2nd 

 molars from above (pi. 34. loc. cit. No. 136) presents a totally dissi- 

 milar aspect. The two ridges into which the worn molar there de- 

 picted is divided, are more oblique thau in Didymodon, while the 3rd 

 molar, which in figs. 13 and 13 a is seen concealed in the alveolus, ex- 

 hibits three distinct ridges. 



Tapir ulus hyracinus, of Gervais, is another . closely allied form. In 

 Gervais' definition of the genus, # he says, " Lower posterior molars 

 with two very distinct transverse ridges, incompletely united by a 

 weak keel perpendicular to their axis, instead of being oblique ; a 

 strong posterior talon ; that of the last resembling a third ridge less 

 large than the two others." The posterior talon of the hinder molar 

 in Gervais' plate (34, 3 and 3 a ) projects far more in a posterior di- 

 rection than the presumed homologous rudiment in the Didymodon, 

 and this difference is observed in a less degree in the preceding tooth. 

 The ridges in Tapirulus, transverse to the tooth's axis, are too well 

 marked to render it likely that they may have been produced by the 

 worn enamel-folds of the denuded cusps in an old Didymodon. 



Hycegulus eollotarsus, of Pomel, which in the other dental charac- 

 ters of its lower jaw agrees with the typical Cainotheria,f differs 

 from them, according to that writer, in the deeper division of the 

 inner points of the second ridge of its lower molars. The figure of 

 the species which Gervais presumes to be identical with Hya?gulus, 

 and names Cainotherium Gourtoisii (pi. 35. f. 4, and pi. 34. fig. 6) dis- 

 tinctly shows a third posterior ridge divided apparently into two 

 cusps to the third lower molar tooth. In Chceromorus, from the ac- 

 cessory cusp of the last molar has a tendency to a ternate division, 

 which is seen in the eroded molar of C. simplex, and more promi- 

 nently in C. mammillatus. In Palaeochcerus, from the accessory cusp, 

 seen laterally, is as high as the two other cusps of the last molar, 

 and even higher than the two median cusps. 



Cainotherium, Xiphodon, Dichodon, and Dichobune, each exhibit 

 the same third lobe to the last molar as in Dichodon,^ repeating the 

 characters of the two previous lobes of the same tooth. In DicJw- 

 bune ovina, this lobe, probably owing to the less degree of wear in 

 the specimens, assumes more the character of an elevated unequal 

 cusp, which, however, as Prof. Owen has pointed out, " plainly con- 



* Loc. cit. p. 56. 



f c Geologist; vol. v. 1862, p. 32 and p. 124. 



t Owen, 'Quarterly Journal of the Geological Society,' vol. xiii. 1S57- pi- iii. fig- 3. 

 The lobe is here marked g. 



