giving twenty-two teeth on each side, or forty-four altogether[110]. The incisors are represented by _i_, the canines by _c_, the premolars by _p_ or _pm_, and the molars by _m_. The numbers above the lines represent the teeth in the upper jaw, those below the lines the teeth in the lower jaw. The milk dentition is expressed by a similar formula with _d_ (deciduous) prefixed to the letter expressing the nature of the tooth. The following terms are of frequent use as characterising certain forms of the grinding surfaces of teeth, and it will be well to define them at once. _Bunodont_ is a term applied to teeth with broad crowns raised into rounded tubercles, e.g. the grinding teeth of Pigs and Hippopotami; _Bilophodont_ to teeth marked by a simple pair of transverse ridges, with or without a third ridge running along the outer border of the tooth at right angles to the other two, e.g. the grinding teeth of _Lophiodon_, Kangaroo, Manatee, Tapir, _Dinotherium_; _Selenodont_ to teeth marked by crescentic ridges running from the anterior towards the posterior end of the tooth, e.g. the grinding teeth of the Ox and Sheep. Teeth whose crowns are low so that their whole structure is visible from the grinding surface are called _brachydont_, while those with higher crowns, in which the bases of the infoldings of enamel are invisible from the grinding surface are said to be _hypsodont_. Bunodont teeth are brachydont, the teeth of the Horse and Ox are hypsodont. Passing now to the appendicular skeleton--the shoulder girdle differs markedly from that of Sauropsids in the fact that the coracoid, except in the Ornithodelphia, is greatly reduced, generally forming only a small process on the scapula. In the pelvis the pubes meet in a ventral symphysis, except in some Insectivora and Chiroptera. In many mammals a fourth pelvic element, the _acetabular bone_, is distinguishable. The ankle joint is _cruro-tarsal_, or situated between the proximal tarsal bones and the tibia and fibula. Carpalia 4 and 5 are united forming the _unciform_; and the ulnar sesamoid bone or _pisiform_ is generally well developed. In the proximal row of tarsal elements there are only two bones, the calcaneum and astragalus. Of these the calcaneum is the fibulare, and the astragalus is generally regarded as the tibiale and intermedium fused[111]. _Subclass I._ ORNITHODELPHIA OR PROTOTHERIA.