450 



oblique abrasion of the enamel, but not carried so far as to expose the dentine ; the 

 links were entire from their origin at the fore and outer angles of the two lobes; the 

 hind surface of the second lobe showed the whole of the angular fossa on its inner half, 

 and the small faint vertical notch external thereto. 



In the subjects of Plate LXXXIX. figs. 1-4, the direction of wear against the upper 

 molar from above downward and backward has extended the abrasion from the second 

 lobe to its hind basal surface, where the bottom of the hind fossa (g) alone remains : 

 with the edges of the wedge-shaped lobes the origins of the link (r)are gone, and only 

 the lower antero-posterior ends remain, dividing the valley into a smaller outer and 

 a larger inner depression. The prebasal ridge (/) is correspondingly reduced in fore- 

 and-aft extent. 



That these changes or modifications of working-surface of m 2 and m 3 are due to age 

 and wear will be clear to any one comparing these teeth in Plate LXXXIX. figs. 2 & 4, 

 with those before quoted of large full-grown individuals of Macropus Titan, described 

 (p. 408) and figured, Plates LXXXI1. & LXXXVI. The size and form of the portions 

 of mandible preserved closely agree with those of the corresponding parts of the more 

 entire mandibles of Macropus Titan, figured there. I here subjoin a view from above 

 (Plate LXXXIX. fig. 1), not before given, to illustrate the correspondence in the breadth 

 and direction of the " ectalveolar groove" (u) and in the thickness of the mandible, 

 including the fore part of the base of the coronoid, with corresponding parts of the 

 bone from the more aged animals that have afforded the subjects of figures 2 & 4, Plate 

 LXXXIX. 



The only difference of note is in the more advanced position of the last molar ; and 

 this is due to the well-known general forward movement of the serviceable teeth when 

 those anterior to them have been worn down and shed — a movement which reaches its 

 maximum in the Elephants. 



A portion of a right mandibular ramus with the last two molars in place and worn 

 (Plate XCVI. fig. 4, and Plate XCVII. figs. 3 & 4) indicates, both by the size of the 

 teeth and that of the jaw itself, a Kangaroo too much exceeding in size the largest indi- 

 viduals of Macropus Titan to belong to that species. A comparison of the above-cited 

 figures with figures 13, 15 in Plate LXXXII. will demonstrate this fact. 



The jaw-bone of Pachysiagon Ferragus is relatively thicker in proportion to the teeth, 

 and the last molar (Plate XCVII. fig. 3, m 3) rises more in advance of the origin of 

 the coronoid process ; but this may relate to the greater age of the individual as shown 

 by the exposure of the dentine in both lobes (ib. fig. 4) of that tooth, though they are 

 not worn to quite the same degree as in fig. 2, Plate LXXXIX. The base of the notch 

 (Plate XCVII. fig. 4, g) remains on the hind part of m 3, and internal thereto are 

 indications of two vertical ridges (Plate CV. fig. 3) which I have not found in any fossil 

 of the genus Macropus. 



§ 20. Leptosiagon* gracilis, Ow. — On similar grounds the present genus and species 



* From Xeirros, slender ; owyutv. jaw-bone. 



