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of extinct Kangaroo are based upon a portion of a right mandibular ramus with m 1 and 

 m 2 in place, part of the socket of m 3, and the whole of that of d <, from which the tooth 

 appears to have been naturally shed or worn away (Plate LXXXIX. figs. 11-15). The 

 transverse fracture of the mandible anterior to d * (ib. fig. 15) shows the bone to be 

 unusually thin in proportion to its depth ; the partial thickening on the inner side is 

 almost ridge-like ; the outer thickening, beginning opposite the inner one, is continued 

 with a convex curve to the lower border of the ramus. 



The distinctive dental character of the present subgenus is in the sculpturing of the 

 hind surface of the molars (ib. fig. 14) ; two slender well-defined pyramids of enamel, 

 in high relief, rise from the base of that surface at its inner half, and terminate in 

 points, the inner one within a line of the unworn summit of the hind lobe, the outer 

 one within two lines of the same. A deep reversed, narrow, angular or pyramidal 

 notch divides the inner pyramid from the inner side of the hind lobe, a deeper notch of 

 corresponding form divides the inner from the outer pyramid, and a fainter, narrow, 

 short, linear groove separates the outer pyramid from the rest of the outer part of the 

 hind surface, which is continued by a bold convexity into the outer side of the molar. 



The outer end of both transverse lobes incline more forward than in Macropus Titan ; 

 but, as in that species, the outer bases of the two lobes meet to define an acute-angled 

 pointed lower termination of the interval or valley there (ib. fig. 11, m 1, m 2). In 

 Sthenurus and Protemnodon the lower termination of the same interspace is rounded, 

 not pointed, the outer bases of the lobes not being in the same degree extended antero- 

 posteriorly. 



There is no accessory ridge from the inner side of the mid link ; but in the smaller 

 outer part of the hollow between the front lobe and the prebasal ridge, defined by the fore 

 link, there is in m 2 a vertical enamel ridge: it is not present in m 1 (Plate LXXXIX. fig. 13). 

 I have seen none such in the corresponding part of any molar of Macropus Titan ; and 

 it may be a variety in the individual specimen exemplifying or indicating the present 

 subgenus. 



But the character of the back part of the molar is decisive, so far as my present 

 observation of existing and extinct Kangaroos has extended, of the taxonomic distinction 

 which I have assigned to Leptosiagon gracilis. 



§ 21. Genus Procoptodon*, Ow. — The indications of this genus, at first fragmentary, 

 have been raised, by evidences successively received since the year 1845, to demonstra- 

 tion, under, at least, three specific modifications. 



The first of these is exemplified, mainly, by maxillary fossils. The most instructive 

 are portions of the right and left maxillary bones, each with three molars (d *, m 1, m 2) 

 and part of a fourth, d 3 (Plate XC. figs. 2-6), of the same skull. 



In antero-posterior extent and in breadth the two molars (m 1, m 2) do not exceed their 

 homologues in the upper jaw of Sthenurus Brehusf; but the configuration of the 



* From 7rpo, before ; kotttw, to pound ; 6Sovi, tooth, 

 t Plate LXXXVII. fig. 6, mi, ma. 



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