TOOTH-GENESIS IN THE CAVIIDA. 267 
of the two conditions, it is easy to imagine that a still further 
stage of degeneration would give the appearance seen in the dog, 
guinea-pig, and Gymnura. 
Just as the rudimentary germ of the premolar is on the eve of 
disappearing, the tooth immediately behind it, the so-called dpm., 
is commencing to appear, but placed much more deeply and 
lying altogether underneath the preceding tooth. Tracing it 
backwards, it is seen to be well-calcified, but has not yet reached 
the surface of the gum. No labial or lingual downgrowths are 
to be seen, though traces of both have been noted in Stage 1. 
There is an interval of 70 sections between the neck of this 
tooth and that of the preceding: consequently I think there can 
be no possible morphological connection between the two. If 
such be the case, then these two teeth can no longer be regarded 
as the morphological predecessor and successor, the one of the 
other. I believe the correct interpretation to be that the more 
anterior tooth is a premolar, probably pm.4, belonging to the 
successional series, and the so-called dpm. is the first true molar. 
The reasons which lead to this conclusion will be discussed below. 
IT shall therefore in what follows speak of the five teeth in the 
upper jaw of the guinea-pig as pm. 4, ms.1, 2, 3, 4. 
The deciduous tooth at this stage (Pl. 26. fig. 1) possesses 
two antero-posterior rows of cusps, and has the appearance of two 
similar portions one behind the other, the anterior being the 
larger. This remark applies to all the cheek-teeth both at this 
and later stages ; and for this reason I think there isa possibility 
of the correctness of the Concrescence theory. The external row 
has three distinct cusps, of which the centre one is the more pro- 
nounced and the posterior slightly smaller than the anterior. 
The internal row also consists of three cusps, the anterior being 
the largest. The middle cusps of the two rows are separated 
one from the other by a wide depression; while the first and third 
cusps of both rows are connected respectively by ridges which 
bound the depression anteriorly and posteriorly. Behind the 
posterior ridge is a second smaller depression separating the 
outer from the inner posterior portions of the tooth which do not 
possess any definite cusps. The anterior and central cusps form 
the anterior larger portion of the tooth; the third cusps with the 
posterior portion of the tooth together forming a miniature of the 
anterior part. The first and third cusps of the exterpal row are 
