No. 2.] THE HARD PARTS OF THE MAMMALIA. 27% 
genus the tubercles slid over each other, and truncated the 
apices until in old specimens they were entirely worn away 
(Fig. 92, c,¢). In Meniscoessus and Stereognathus we have an 
interesting illustration of the effect of the action of cusps on 
each other when under prolonged mutual lateral thrust. Their 
external sides have been drawn out into long angles in the 
direction of thrust, converting their transverse sections from 
circles to crescents. As the thrust is in the Multituberculata 
longitudinal, the crescents are transverse to the axis of the jaw. 
In the selenodont Diplarthra, where the thrust is transverse to 
the line of the jaw, the crescents are longitudinal. That similar 
effects should accompany similar movements in two groups of 
Mammalia so widely separated as these two is strong evidence 
in favor of the belief that the two facts stand in the relation of 
cause and effect (Fig. 93, Figs. 4 and 2). 
IV: CONCLUSIONS. 
Summarizing the preceding investigations, the structure of 
the Mammalian skeleton and dentition may be referred broadly 
to the two general classes: excess of growth, and defect of 
growth. Each of these may be again divided into two series, 
as follows :— 
Excess of growth, oes 
) Luxuriance. 
Disuse. 
Defect of growth, Poveree 
Progressive evolution results principally from the first two 
conditions, which have frequently codperated in the develop- 
ment of structures. These may be classified as effects of the 
following mechanical causes : — 
A. Motion in Articulation. 
1. Impact only, or chiefly. 
Facetting of distal end of radius in Diplarthra. 
Expansion of proximal end of radius in Diplarthra. 
External trochlea of metapodials in Diplarthra. 
Grooving of distal end of tibia by astragalus. 
Grooving of proximal end of astragalus by tibia. 
