276 DR. H. W. MARETT TIMS ON 
anterior premolar, but slightly backward, closely appressed 
to the second deciduous, so that with its anterior moiety it 
covers only the posterior part of the premolar ; besides it could 
not possibly cover the latter completely, being much smaller.” 
And, as he points out, Fraas [9] states that the anterior pre- 
molar in Prolagus has no deciduous predecessor, but that it 
“comes into place through the same lacuna.’ Clearly, then, 
this is not a unique condition in the Rodents, and I have referred 
[22] to what I believe to be a somewhat similar condition in 
Canis. Regarding the deciduous tooth as the first molar, its 
successor in position would be pm.4. Connected with the latter 
is a “ concentric body.” Though this tends to support my con- 
tention, too much weight must not be attached to it, since a 
similar structure has been referred to in relation to the posterior 
teeth in Cavia. 
Another question opens up: If these “‘ concentric epithelial 
Lodies” are really tooth-vestiges, since they are found in the 
posterior part of the jaw, either the posterior cheek-teeth must 
be regarded as premolars, or else these bodies are the vestiges of 
deciduous molars, and therefore the usually accepted distinction 
between molars and premolars breaks down. I am inclined to 
the latter opinion, having always held that the molars belong to 
the permanent series. If this be a correct interpretation, then 
in this particular the Rodents retain an extremely primitive 
condition ; and the statements made by Fraas with regard to the 
tooth-change in Prolagus may not be far wrong, and certainly 
do not merit the unfavourable comments which have been made 
upon them. 
It will have been seen that the cheek-teeth above described arise 
as a single Primitive cone to be soon followed by the appearance 
of external and internal cingula. As the tooth elongates, two 
antero-posterior rows of cusps arise; the primary cone becoming 
the median-external in position and the largest in size. The 
anterior and posterior cusps of each row respectively become 
united forming transverse ridges, the median cones remaining 
separated by a cleft. The anterior cusp and median cone of the 
outer row together with the anterior transverse ridge form a 
crescentic edge, and this gives rise to the anterior moiety of the 
adult tooth. The median-internal cone disappears. The postero- 
internal cusp, together with a subsequent backward extension of 
the end of the tooth, forms the posterior part of the adult tooth, 
