CURVE OF SPEE IN MAMMALS 
167 
normal relation to any great extent and I have therefore employed 
the method throughout the course of this investigation. For con- 
venience of measurement the buccal cusps of the lower teeth and 
the middle point of the anterior face of the articular surface of the 
condyle have been designated individually from before backwards 
by letters of the alphabet, thus: a for first bicuspid, h for second 
bicuspid, c, d, e for first molar, /, g for second molar, h, i for third 
molar, and K for condyle {fig. 1). 
There are several mammals to which the same designation caimot 
be applied on account of differences between their dental formula 
and that of man. Still another exception is found in the case of the 
opossum, owing to the unequal length in that form of the mesio- 
buccal cusp and disto-buccal cusp of the molar; the latter being two- 
thirds the length of the former. When the teeth are in occlusion, 
the disto-buccal cusps of the lower teeth are in contact with their 
antagonists, while the mesio-buccal cusps enter between the two 
cusps of the upper teeth. I therefore made the measurement only 
on the disto-buccal cusps, because in such instances it appears to me 
more reasonable to take the cusps which give contact with their an- 
tagonists, since these cusps alone have real importance in the process 
of occlusion. The lower jaw was placed on a straight line drawn on 
the table in such a way that all of the cusps of these teeth are on that 
line when seen from above. Then the distances between a and h, 
h and c, etc., were directly measured to 0.1 mm., the points of the 
callipers being held in a line parallel to the base-line on the table 
(Jig. 1). As for the distance between i and K, I measured this holding 
the points of the callipers parallel to the projection plane {QP), but 
not necessarily parallel to the horizontal plane (or base-Une) . These 
measurements were entered as the abcissae on a sheet of paper which 
represents the projection plane. 
The values of ordinates corresponding to the abcissal values were 
determined by the following method. The jaw was placed on the 
table, and the distance between the cusps and the table were measured 
to 0.1 mm. by sliding a cross-bar, which is attached to a rod standing 
perpendicular to the table, until it touched the top of the cusp. The 
distances for the cusps as thus measured were entered as ordinates 
on the same sheet of paper. 
