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itself first of all as a retardation, the posterior members of the fourth 
dentition being developed later than the anterior members of the fifth 
(upper jaw). In the same way, some of the anterior teeth of the 
alternating series have been displaced before the posterior members 
have made their appearance. It is worthy of note that retardation 
increases the size of a tooth. This is especially conspicuous in the 
case of the posterior alternating teeth in the maxilla, which have been 
much retarded in development as compared with the anterior members 
of this series. 
Concrescence. 
Before passing on to the consideration of the incubation period, 
it will be well to discuss a process which is of both general and 
special interest. During the last few weeks before hatching, a 
number of the alternating teeth of the lower jaw become fused with 
each other. This fusion does not affect the first three, or (in the 
specimen I examined) the posterior teeth, but from the relations of 
the enamel organs at this stage, and the condition of the teeth at 
later stages, there is no doubt that all the alternating teeth in the 
lower jaw (except perhaps the first three) become fused together. It 
is difficult to distinguish the limits of the individual enamel organs 
where this fusion is in progress, but they appear to remain to some 
extent independent (Fig. 5). The pulp cavities of the fully formed 
teeth do not communicate directly with one another. We have here 
a clear case of concrescence, which results not in the formation of a 
true multicuspid tooth, but rather of a serrated dentinal ridge. The 
fusion is so far from being advantageous to the teeth affected, that 
they are thereby restricted in their growth, and become worn away 
at an early age. ‘They are also prevented from adapting themselves 
to the increasing length of the jaw, and new teeth (the uniform series) 
come up from behind, as we have seen (Figs. 2, 3 and 4) to functio- 
nally replace them. The lower jaw is the only region in 
which I have found concrescence, and the structure of 
thelarge anterior “teeth” is not dueto this process, but 
mainly to overgrowth and outgrowth of bone. 
The number of animals in which concrescence has been shown 
to occur is not very great, and this instance may be of some inter- 
est to the upholders of the Concrescence Theory of cusp development. 
Röse (3) describes the tricuspid teeth of Chamaeleo vulgaris as 
being produced by concrescence, and uses his results as an argument 
in favour of the theory. KÜKENTHAL (4) also lays great stress on 
the fusion of teeth of two dentitions, a process he found to occur 
