OSTEOLOGY OF THE IIYOPOTAMID.H 
of the Suina. As this inferior extremity of the humerus seems to me a very important 
part, especially as showing the modifications which a bone may undergo in adaptation 
to different conditions of life and organization, I must describe it at length. 
Looking at the distal extremity of the humerus in the two series of fossil Ungulata, 
we shall see that its shape is at first exceedingly and typically different in Paridigitata 
and Imparidigitata. In the oldest Imparidigitata, as Palceotherium , and even in the 
living Rhinoceros, the distal extremity of the humerus is quite hourglass-shaped — that 
is, it looks as if two truncated cones were joined together by their apices. At the point 
of meeting we have a middle groove, from which the two horizontal cones thicken 
gradually in both directions, inwards and outwards. As we have every reason to 
suppose that An op loth erium is the descendant of a very old type of Paridigitata, we may 
look to its humerus as giving us the typical form of the distal extremity of this bone 
in old Paridigitata. Now the distal extremity of the humerus of Anoplotherium is 
totally different from what we have seen in Palceotherium ; the difference may be best 
imagined if we say that in lieu of the middle groove, where the two cones meet, we 
find in Anoplotherium a round bulging, which goes all round the distal extremity of the 
humerus (see Blainville, Ost. Anopl. pi. iii.). This middle bulging is very characteristic of 
all mammals in which the humerus is very movable upon the two bones of the antibra- 
chium — so in Man, in most Carnivora, and in those liodents which use their fore paws as 
hands. With the reduced mobility of the humerus upon the antibrachium, we remark 
a concomitant change in its distal extremity ; the middle bulging recedes gradually to 
the outer half of this extremity and becomes much sharper, till at last, in animals with 
greatly reduced limbs whose humeri are fitted only for a simple sliding movement in 
one vertical plane (as in our modern Ruminants, and in some Rodents, as Hares), this 
round bulging of the Anoplotherium is reduced to a sharp ridge, which enters deeply 
into a corresponding groove on the proximal extremity of the radius (which must 
necessarily be adapted to all the modifications of the humerus). It is exceedingly 
interesting to follow this gradual modification, step by step, through all the inter- 
mediate stages presented by the Clicer other ium of Lartet, the Hyomoschus crassus , to our 
modern Ruminants, whose humeri show, instead of the Anoplotheroid bulging, an 
exceedingly sharp ridge fitting closely into a corresponding groove of the radius, and 
preventing any other movements save those in one vertical plane. It is interesting to 
notice that we see the same change going on in the Imparidigitata, though their 
starting-point, from the simple hourglass-shaped form of the humerus of the Palceo- 
theridee , is so different. In this series a rising appears gradually on the outer half-cone : 
in Palceotherium medium ; this rising goes on increasing with the gradual reduction 
of the free movements of the fore limb in Anchitherium and Ilipparion , till it reaches 
the state iii which we now find it in the Horse ; and in this last the distal extremity of 
the humerus is nearly like that presented by Ruminants ; but as I have discussed this 
case in my memoir on Anchitherium *, I will not return to it in this place. 
* Mem. Acade'mie cle St. Petersbourg, 1S73. 
