68 
DR. W. KOWALEVSKY ON THE 
in the two groups of the didactyle and tetradactyle Hyopotamidee of the oldest Miocene 
and Upper Eocene. 
What, then, are the relations of these lateral digits to the whole manus and pes, 
considered in their primitive unreduced condition. The inner, or the second lateral 
digit of the manus is supported by the trapezoid ; it is not, however, limited only 
to this bone, but leans a little over the inner edge of the third metacarpal, and goes to 
touch the os magnum, which presents a special facet to this second metacarpal. We 
may see this typical relation in the second digit of the Anoplotlierium tridactylum 
(Plate XXXVII. fig. 2), in the rudiment of the second metacarpal in the Paris Anoplo- 
tlierium from the gypsum (see De Blainville, Anopl.), and in the very complete fore 
foot of Hippopotamus and Hyopotamus (Plate XXXVII. figs. 1 & 20, n. t, m). In the 
posterior limb, the inner or second metatarsal is supported entirely by the second cunei- 
form (the homologue of the trapezoid), and by the tibial edge of the great or third 
cuneiform(the homologue of the os magnum), as maybe seen in Hippopotamus , figs. 9 & 10, 
Anoplotlierium , fig. 11, Hyopotamus (Plate XXXVIII. fig. 2), and Anthraco therium * . 
The outer or fifth digit of the manus and pes in all Ungulata Paridigitata (and, in 
fact, in all existing Mammalia) is supported by the same carpal bone as the fourth. 
There is not a single exception to this rule ; and in all Mammalia, wherever the fourth 
and fifth digits of the manus and pes are present, they are always supported by one bone 
— the unciform in the manus, and its homologue (the cuboid) in the pes. 
Seeing that the full number of digits in Mammalia is five, while the number of carpal 
and tarsal bones which give support to them never exceeds four — three inner digits 
being supported each by a separate bone, while the two outer are always supported by a 
single carpal and tarsal bone — the question may arise, is this relation primitive for all 
Mammalia 1 or is it a result of coalescence of two outer carpal and tarsal bones into one 1 
As we have not the slightest notion of the skeleton of the first mammals, nor even of the 
geological time when they made their appearance, the solution of this question is not 
to be expected now. Turning to some of the Amphibia and Beptilia with completely 
developed digits, we generally, or at least very often, find, as has been proved by the 
beautiful researches of Gegenbaur, that each of their digits in the manus and pes is 
supported by a special carpal and tarsal bone, there being five distinct bones in the 
second row in the manus and pes. Now it is very possible that the first progenitor of 
the class Mammalia had also five carpal and tarsal bones in the second row, or, what is 
more probable, seeing that there is not a single instance of these two outer bones being 
found separate in any mammal, living or fossil, that their coalescence was effected 
before the evolution of the first truly mammalian type, and that they passed into this 
class already coalesced. 
I have already mentioned that, beginning with the mammals of the oldest known 
Eocene deposits, the oldest mammals of whose skeleton we are able to form a tolerably 
clear idea (the oldest Mammalian remains, though proving the existence of the class as 
* This relation is very characteristic, not only as regards Ungulata, but nearly all Mammalia. 
