448 Recent Literature. [July, 
or reciprocating movement, parallel to the medial axis or line; 
the fourth is that which combines to a great extent the second 
and third. It is also to be observed that the lines described in 
most cases are not straight, but only approximately so, they 
being more or less curvilinear. With these four distinct types of 
mandibular movement there are as many distinct types of tooth- 
modifications, to which almost all the forms of teeth of Mamma- 
lia yet known may be referred. e bunodont,type is that char- 
acteristic of group 1, and in which the tubercles have not been 
modified, because there have been no movements of a proper 
kind to produce the modifications in the enamel foldings as 
observed in the longitudinally or the transversely folded enamel 
crowns of the lophodonts (ridge or hill tooth). To this foregoing - . 
group I belong also all the haplodont type (simplest form) as 
observed in the toothed whales. : 
According to the doctrine of mechanical dental differentiation, 
the foldings of the enamcl in the teeth of groups 2, 3 and 4, 
whether they be longitudinal or transverse, are due either to the 
lateral or reciprocating movement of the jaws. The sub-group, 
selenodonts—so called in consequence of the crescent-shaped 
foldings of the enamel of which the deer is a familiar example—- 
is perhaps the most striking illustration of all. The character- 
istic crescent-shaped tubercles, the author thinks, have probably 
been evolved by a very slow process of flattening and bending 
outwards or inwards of the cornu of the tubercles, due to the 
strains exerted in masticating the tough woody food. The move- 
ment in this case being constantly in one direction, makes it still 
more probable that such has been the history of the process. — 
e incisors of many animals having been lost or hypertrophied 
either from the assumptions of their functions by other parts, 
as the lips, tongue or trunk, or by substitution of another func- 
tion, as in the case of the tusks of the Proboscidians, it would 
seem that mechanical resistance has much to do with their spẹ- 
cial development, as is also seen in Rodents. 
A summary of the foregoing views the author has stated as 
follows . oe 
“1, That the earliest and simplest type of mammalian jaw- 
movement was that in which the mouth was simply opened and 
closed, without mandibular excursion, and co-existent with the 
simple haplodont or bunodont molar. ; 
~ “2. That the development of the various kinds of excursive 
mandibular movement has apparently been progressive. —— — 
“ 3. That as the excursive movements have increased in ine ; 
plexity, there has been an apparent increase in the complexity 0! — 
the enamel foldings, ridges and crests. : 
“4. From the fact that the foldings, etc., have apparently been 
modified in conformity to the ways in which the force used in 
‘mastication was exerted, it is concluded that the various moces 
. 
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