102 THE ESSEX NATURALIST.
fossil from Harwich, thus started the idea that the Lower Eocene
mammals possibly included the common ancestors of the hoofed
mammals. Comparison with Upper Eocene fossils suggested
that the five-toed Hyracotherium might actually be the fore-
runner of the one-toed horses. Later discoveries, especially in
the Eocene of North America, have confirmed this impression,
and the researches of J. Leidy, E. D. Cope, O. C. Marsh, W. B.
Scott, H. F. Osborn, J. L. Wortman, and others, have made
known many allied hoofed mammals which seem to belong to
the ancestral stem.
It now appears that all the early Eocene mammals were
characterised by a comparatively small and simple brain, which
gradually enlarged and improved in their successors of later
Tertiary periods. All of them had a skull of nearly the same
shape, whether flesh-eaters, insect-eaters, or vegetable-feeders.
Many of them—if not the majority—also retained a stout and
powerful tail inherited from their reptilian ancestors. The
hinder end of the body would gradually taper into the tail, as it
still does in such primitive mammals as the thylacine (Thylacinus)
from Tasmania and the aard vark (Orycteropus) from South
Africa. It is wrong to restore early Eocene mammals with the
appendicular tail of a dog or a cow.
The teeth of these forerunners of the typical modern mammals
are also noteworthy. Whereas the teeth in reptiles are variable
in number, they are always 44 (or 22 in each jaw) in the early
Eocene typical mammals, namely, three incisors, one canine,
four premolars, and three molars in each half. The later land
mammals only differ from these in various reductions in the
primitive 44 ; while a few sea-mammals are exceptional in
having the number increased. Not only is there this curious
fixed arrangement of the teeth in the typical mammals, but the
tubercles or cusps on the grinding teeth are individual entities
which can be identified and traced through long pedigrees in
geological time. These tubercles are characteristic of all of
them—not merely of the "Hog tribe." In some, the tubercles
are pointed for flesh-eating or insect-eating ; in others they
gradually pass more or less into ridges effective for grinding
succulent plants. There can be no doubt that in most—if not
all—of the still earlier really ancestral (Mesozoic) mammals,
each grinding tooth had three such tubercles or cusps. Some of