TOOTH-GENESIS IN THE CAVIIDR. © 2G 
while the narrow band connecting the anterior and posterior 
portions represents the posterior transverse ridge. In the 
younger stages the teeth are decidedly more multitubercular 
than in the adult. Compare fig. 2 (Pl. 26) with the adult teeth 
of Cavia as shown in fig. 7 B. 
In the posterior molars a further small postero-external 
extension of the tooth arises in the form of a rounded process. 
It is also present, though to a much less extent, in the anterior 
cheek-teeth. Itis very pronounced and plicated in the posterior 
molar of Dolichotis, and its size forms the principal difference 
between the molars of this fossil rodent and those of the existing 
Caviide (Pl. 26. fig. 7). 
It may here be noted that the root of the tooth is frequently 
seen to be lateral in position, as shown in Pl. 26. fig. 10. This is 
not usually so marked in other animals, in which the obliquity 
of the adult teeth is not present to the same extent as in the 
Rodents. 
Within the limits of the Mammalia comparative odontologists 
have referred to the existence of fowr distinct dentitions—a 
Pre-milk, a Milk or Deciduous, a Permanent or Successional, and 
a Post-permanent. The existence of all four at one and the same 
time has not, so far as I am aware, been shown to be present 
in the same animal. The Pre-milk dentition is said by Leche 
and others to be present in the Marsupials, but such an inter- 
pretation of the vestigial representatives, such as undoubtedly 
occur in Myrmecobius, Phascologale, Dasyurus, and others, 
depends upon the functional teeth of the Marsupials representing 
the true milk dentition. I have previously [21] expressed my 
belief that another and more probable explanation is forthcoming, 
and in this opinion I am supported by Wilson & Hill [24] 
and by Tomes [23]. I would regard therefore Leche’s vestiges 
as remains of a deciduous dentition. The evidence as to the 
existence of traces of a Post-permanent dentition in many 
mammals is, I think, undoubted: they have been described in 
Man, Seal, Hedgehog, and Dog; and if my interpretation be 
correct, it is also to be found in Kikenthal’s lingual down- 
growths of the dental lamina as described by him in Didelphys 
and in the Cetacea. 
In the Rodentia there are well-marked evidences of at least 
two dentitions—the milk and permanent, though the former 
seems tending to disappear. 
LINN. JOURN.—ZOOLOGY, VOL. XXVIII. 20 
