> 286 DR. H. W- MARETT TIMS ON 
The same may be said with respect to the Hystricide. The 
diagram (Pl. 26. fig. 8) shows the outline pattern of Stichomys, 
Spaniomys, and the recent Hystria leweura (Camb. Zool. Mus.), 
fig. 7, p. 283. As the teeth are much worn, no information 
can be obtained as to the original disposition of the tubercles ; 
nevertheless, the outline-pattern, though simpler, is sufficiently 
similar as to suggest a possible line of descent. 
Though it is difficult to obtain any decided results from a 
comparison of the fossil teeth, owing to the wearing-down to 
which they have been subjected, still it seems evident that the 
complexity of the molars, which is undoubtedly more common in 
the existing forms than in the earlier ones, is due to the external 
and internal plications of the enamel rather than to the develop- 
ment of new tubercles. These animals, which are undoubtedly 
Rodents with the characteristic dentition and molar pattern, 
extend back to the Inferior Eocene. The Tillodontia are not 
found before the Lower and Middle Hocene, at which period, as 
we have seen, typical Rodents are present. It is difficult to 
conceive that the well-developed canines should have disappeared 
so rapidly and so suddenly together with at least two premolars, 
and that the incisors, which are only “becoming scalpriform” 
[26] in the Tillodontia, should have so quickly developed, if we 
are, with Cope, to regard them as the ancestors of the Rodents. 
And again, the Rodent molars have already assumed their charac- 
teristic pattern, whereas the molars and premolars of Tillotheriwm 
are “distinctly tritubercular, while those of Hsthonyx are quite 
unlike any Rodent molars” [4]. There is also the fact that the 
humerus in the Tillodontia possesses an entepicondylar foramen, 
which is not present in any existing Rodent. On the other hand, 
some of the Multituberculata are considerably older than the 
earliest known fossil Rodent, extending back into the Jurassic 
Period. In them the canines and several of the premolars have 
already disappeared, the incisors reduced in number, one being 
large and functional, and the pattern of the cheek-teeth in some 
instances approaching even in some degree to the unworn teeth 
of the existing Hystricide. 
