THE SKELETON 33 
ml, pl). Finally, in the bunodont series, the addition of a postero- 
internal cusp (Fig. 5, hy), termed the hypocone, forms the sextuber- 
cular molar. 
The following table exhibits, in a collective form, the names 
and relations of all the above-mentioned cusps, and the letters by 
which they are indicated in the figures :— 
Upper Monars. 
Antero-internal cusp =protocone =pr. 
Postero ,, or 6th cusp =hypocone =hy. 
Antero-external cusp =paracone =pa. 
Postero,, s =metacone =me. 
Anterior intermediate cusp = protoconule = ml. 
Posterior 35 oF =metaconule = pl. 
Lower Motars. 
Antero-external cusp =protoconid =prd. 
Postero ,, 5 =hypoconid =hyd. 
Antero-internal or 5th cusp =paraconid =pad. 
Intermediate (or in quadritubercular 
molars antero-internal) cusp. =metaconid =med. 
Postero-internal cusp =entaconid =end. 
The common occurrence of trituberculism in the mammals of 
the earlier geological epochs is, as remarked by Osborn, very 
significant of the uniformity of molar origin. Thus, among the 
Mesozoic mammals (with the exception of the group known as 
Multituberculata, in which the molars are constructed on a different 
type), trituberculism occurs in the great majority of the genera; 
while out of 82 species, belonging to five different suborders from 
the Lowest or Puerco Eocene of the United States, all but four 
exhibit this feature ; and the same holds good for the mammals of 
the corresponding European horizon. At the present day trituber- 
culism persists in the Lemuroidea, Insectivora, Carnivora, and Mar- 
supialia. In the Carnivora there is a tendency to lose the meta- 
conid, while in the bunodont molars of the Ungulata it is the 
paraconid that disappears. 
Ill. THE SKELETON, 
Definition.—The skeleton is a system of hard parts, forming a 
framework which supports and protects the softer organs and 
tissues of the body. It consists of dense fibrous and cartilaginous 
tissues, portions of which remain through life in this state, but the 
greater part is transformed during the growth of the animal into 
bone or osseous tissue. This is characterised by a peculiar 
3 
