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 ApJielotJierium Duvernoyi, Gerv., derived from the Paris gypsum, 
which bears many curious points of resemblance to Didymodon, and 
W'hich 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 than in Didymodon, while the 3rd 
molar, which in figs. 13 and 13^ is seen concealed in the alveolus, ex- 
hibits three distinct ridges. 
Tapir ulus hyr acinus, 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 (31, 3 and 3^) 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 collotarsus, of Pomel, which in the other dental charac- 
ters of its lower jaw agrees with the typical Cainotheria,t 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 Hysegulus, 
and names Cainotherium Courtoisii (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 Choeromorus, 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 Palaeochoerus, 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 Dicho- 
hune 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. 
t ' Geologist,,' vol. v. 1862, p. 32 and p. 124. 
X Owen, 'Quarterly Journal of the Geological Society,' vol. xiii. 1857. pi- iii. fig- 3. 
The lobe is here marked g. 
