CURVE OF SPEE IN MAMMALS 
199 
of Spec, but is a straight line (see section II). We thus see that, in 
some of the mammals, the curve of Spec is found irrespective of the 
presence or absence of the antoro-posterior movement of the lower 
jaw during mastication. It naturally follov/s that the close relation 
said, by Spec, to exist between the curve of Spee and the antero- 
posterior movement of the lower jaw, cannot be accepted without 
modification. 
From the above it is clear that the statements made by Spee that 
there is close relation between the curve of Spee and the form of fossa, 
or between the curve of Spee and the antero-posterior movement of 
the lower jaw during mastication, cannot be accepted, whether we 
consider them from the standpoint of the mechanism of mastication 
or from actual tests made on the skull. Moreover, it is clearly in- 
dicated by this series of facts that the movements of the lower jaw 
during mastication in all animals named by Spee, as well as those 
studied by me, are not the antero-posterior movement only, but 
rather a mixed motion resulting from a combination of the three 
components which have been recognized. It follows, therefore, that, 
in the animals in which the curve of Spee exists, the teeth do not have 
any greater efficiency as a masticatory organ than in those animals 
which do not possess the curve of Spee. 
It may be pointed out that, although the curve of Spee is often 
called "the compensating curve" under the erroneous impression 
that it compensates the movement of the jaw during mastication, yet, 
as I have already indicated in the preceding section, such compen- 
sation, in the sense of Spee, does not take place, and for this reason 
the use of the term should be discontinued. 
X. SUMMARY 
The main results of the present investigation are indicated in the 
following findings: 
1. The hne touching the summits of the buccal cusps of the bi- 
cuspids and molars was carefully studied on the skulls of numerous 
mammals. According to the nature and form of this curved line, 
three types may be distinguished; namely, (a) a curved line which 
corresponds to the arc of a circle with its convexity downwards (the 
