PRIMITIVE MAMMALS IN THE LONDON CLAY CF HARWICH, 103
these survived until the Eocene period, but by that time most of
the grinding teeth had acquired a fourth tubercle, and minor
tubercles or cusps were beginning to occupy intervening spaces.
In Coryphadon the three cusps and a supplementary fourth were
being deepened and extending into ridges. In Hyracotherium
the tubercles were still low ; many of them being nearly conical,
and others with only a slight tendency to a crescentic or ridged
shape. In the immediate successors of Hyracotherium crescents
or ridges were developed, and the basis or framework of the tooth
of a rhinoceros or a horse clearly arose. A study of the specimen
of the typical H. leporinum (=Pliolophus vulpiceps) from
Harwich shows that in pattern its dentition is still the most
primitive in the horse-series10 that has been discovered. Further
information about the rest of the skeleton would therefore be
most welcome—especially about the presumably five-toed hind
foot. By the end of the Eocene period some of the successors
of Hyracotherium had already become three-toed. The early
stages in this progression are so interesting that every effort
should be made to recover them.
More European specimens of the Lower Eocene mammals are
particularly needed for comparison with the numerous American
discoveries, because it is now almost certain that during the
Tertiary epoch the lands of the Old and the New Worlds were
sometimes united, sometimes separate. It would be interesting
to know which of the groups are of single origin and which have
developed independently on two parallel lines. As the whole
history of modern mammals will be revealed by Tertiary fossils
when they are sufficiently well known, there is opportunity here
for shedding light on many of the fundamental problems of
organic evolution.
EXPLANATION OF THE PLATE.
Hyracotherium leporinum, Owen (= Pliolophus vulpiceps, Owen) ;
photographs of skull in upper view, (1) ; left side view, (2) ; and right side
view, (3) ; somewhat reduced.—London Clay ; Harwich. Specimen in
the British Museum (Nat. Hist.)
10 C. Earle, Amir. Naturalist. 1816. p. 131 ; J. L. Wortman, Bull. Amer. Mus. Nat. Hist.,
vol, viii. (1896), pp. 81-103 ; C. Dsperet, Bull. Soc. Geol. France, ser. 4. vol. i. (1901), pp. 199-323,