MEG ATHERIOIDS.

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the lower jaw: they are more closely arranged, are longer, and moredeeply implanted than in the smaller Megatherioids ; they presenta more or less tetragonal figure, and have the grinding surface tra-versed by two transverse angular ridges. Plate 83 exhibits a longi-tudinal section of the five molars of the upper jaw, in situ; anddemonstrates the great extent of the persistent pulp-cavity; thenatural length of the series is ten inches.

The first or anterior molar is the second in point of size,the last being the least, as in the Scelidothere. The first molaris eight inches and a half in length ; the pulp-cavity extends fiveinches from the base : it presents two slight curvatures, one havingthe convexity turned forwards, and the other inwards. The trans-verse section gives an irregular semicircle, with the convexity turnedforward, and the flat side next the second tooth; the angles at whichthis side joins the curve are rounded ; the outer angle is some-what produced, and the outer side of the curve is flattened. Thecentral axis of the tooth, formed by the vascular dentine, is irre-gularly tetragonal, the cement is thick on the anterior and posteriorsurfaces, thin on the sides of the tooth.

The second molar is the largest of the upper series ; it exceedsnine inches in length, is of a tetragonal form, with two slightcurvatures, as in the first molar. The posterior and broadest sideis nearly flat, the anterior side somewhat convex, the outer andnarrowest side is concave, the inner side is sinuous, having a medianlongitudinal eminence between two longitudinal concavities. Thecentral axis of vascular dentine is more compressed from beforebackwards, than in the preceding tooth, and its posterior surfaceis concave ; the two transverse ridges of the grinding surface ofthe tooth are nearly equal; but the sloping side formed by thedentine, is larger than that formed by the cement.

The third tooth is of nearly the same size and form as thesecond; but is somewhat narrower, the anterior or outer angle isless rounded off, and the external longitudinal depression is deeper.

The fourth molar is smaller than the two preceding, but ofneairly equal length, viz. eight inches and a half, and is distinguishedfrom all the other teeth, by being curved in only one direction,

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