268 DR. H. W. MARETT TIMS ON 
slightly undermined at their bases, representing the involution 
of enamel which is carried to such extremes in the teeth of some 
other Rodents. 
The second molar, or second cheek-tooth of the adult dentition, 
resembles the anterior part of the tooth just described. There 
are two external cones, the posterior being of considerable size, 
the anterior inconspicuous, and the same may be said of the two 
internal cusps. The antero-internal and antero-external cusps 
are partially fused with each other transversely, whereas the two 
posterior cusps are separated by a deep but narrow cleft (Pl. 26. 
fig. 8). The posterior part of the tooth is made up of a mass of 
considerable size with a rudiment of a cusp, both internally 
and externally, the latter being slightly the larger. At the base 
of the tooth, on both its outer and inner aspects, is a well- 
marked rounded prominence which I think must be regarded 
as the cingulum. 
The third molar is not calcified. It presents a broad, trans- 
versely elongated surface with an external and an internal cusp, 
the former being the larger. ‘here is a well-marked lingual 
downgrowth of the dental lamina. In connection with this tooth 
there is one of the concentric epithelial bodies to which I have 
already referred. 
If these bodies really do represent the last stage in the dis- 
appearance of a tooth, we have here in connection with an 
undoubted molar tooth evidence of three dentitions, from the 
central one of which the permanent tooth developes. Adopting 
the line of argument I have previously used when referring to 
the Marsupial dentition [21], it would seem to show that the 
molar teeth do belong to the successional series—a view which, 
though held by many, is not universaily accepted. The fourth 
molar is present in a very rudimentary condition. 
The second, and last stage of Cavia cobaya examined by Adloff 
had a head-length of 8 cm. This measurement corresponds 
exactly with my Stage 3; but from the description given, it is 
evident that Adloff’s was a much younger specimen, the difference 
being no doubt due to a difference in the method of measuring. 
From a comparison of the results, I am inclined to think that 
his specimen must have been slightly younger than my Stage 2. 
He finds that the first “Anlage” in the hinder portion of 
the jaw is that of the premolar of the first dentition, that is of the 
deciduous tooth. This is in agreement with what I have found 
