Antero-internal cusp = protocone = _pr_. Postero ” or 6th cusp = hypocone = _hy_. Antero-external cusp = paracone = _pa_. Postero ” ” = metacone = _me_. Anterior intermediate cusp = protoconule = _ml_. Posterior ” ” = metaconule = _pl_. LOWER MOLARS. Antero-external cusp = protoconid = _prd_. Postero ” ” = 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 trituberculism persists in the Lemuroidea, Insectivora, Carnivora, and Marsupialia. In the Carnivora there is a tendency to lose the metaconid, while in the bunodont molars of the Ungulata it is the paraconid that disappears. III. 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 histological structure and chemical composition, being formed mainly of a gelatinous basis, strongly impregnated with salts of calcium, chiefly phosphate, and disposed in a definite manner, containing numerous minute nucleated spaces or cavities called lacunæ, connected together by delicate channels or canaliculi, which radiate in all directions from the sides of the lacunæ. Parts composed of bone are, next to the teeth, the most imperishable of all the organs of the body, often retaining their exact form and internal structure for ages after every trace of all other portions of the organisation has completely disappeared, and thus, in the case of extinct animals, affording the only means of attaining a knowledge of their characters and affinities.[8]