266 



MARSUPIALIA. 



Leyden Museum, the canines have a longitu- 

 dinal groove on the outer side. 



The characteristic form of the trenchant pre- 

 molar has just been alluded to : its maximum 

 of development is attained in the arboreal Po- 

 toroos of New Guinea ( Hypsiprymnus ursinus 

 and Hyps, dorcocephalus), in the Tatter of which 

 its antero-posterior extent nearly equals that 

 of the three succeeding molar teeth. In all the 

 Potoroos the trenchant spurious molar is sculp- 

 tured, especially on the outer side, and in 

 young teeth, by many small vertical grooves. 

 The true molars each present four three-sided 

 pyramidal cusps ; but the internal angles of 

 the two opposite cusps are continued into each 

 other across the tooth, forming two angular or 

 concave transverse ridges. In the old animal 

 these cusps and ridges disappear, and the grind- 

 ing surface is worn quite flat, as in Jig. 91, 

 which represents the dentition of the original 

 Potoroo, described in White's Voyage. 

 Genus MACROPUS. Kangaroos. 



In the genus Macropus (Jig. 92) the normal 

 condition of the permanent teeth may be ex- 

 pressed as follows : 



Incisors, 



3 3 

 1 1 



canines 





 



premo- 



lars, - ; molars, - = 28. 

 1 1 4 4 



The main difference, as compared with 

 siprymnus, lies in the absence of the upper 

 canines as functional teeth ; the germs, how- 

 ever, of these teeth are always to be found in 

 the young mammary fcetus of the Macropus 

 major, and I have seen them present, but of 

 very small size, and concealed by the gum, in 

 the adults of some small species of kangaroos, 



Fig. 92. 



Macropus major, one-third nat. sixe 



as Macropus rufiventer, Ogilby, and Macr. 

 psdopus, Gould. This, however, is a rare ex- 

 ception ; while the constant presence and con- 

 spicuous size of the canines will always serve to 

 distinguish the Potoroo from the Kangaroo. 

 But there are also other differences in the form 

 and proportions of certain teeth. The upper 

 incisors of the Macropi have their cutting 

 margins in the same line, the anterior ones not 

 being produced beyond that line, as in the Hyp- 

 siprymni : the third or external incisor is also 

 broader in the kangaroos, and is grooved and 

 complicated by one or two folds of the enamel, 

 continued from the outer side of the tooth ob- 



liquely forward and inward. In most species 

 the anterior fold is represented by a simple 

 groove : the relative size of the outer incisor, 

 the extent and position of the posterior fold of 

 enamel, and consequently the proportions of 

 the pait of the tooth in front or behind it, vary 

 more or less in every species of Macropus : 

 there are two folds of enamel near the anterior 

 part of the tooth in Macr. major, and the pos- 

 terior portion is of the greatest extent, and the 

 entire crown of the tooth is relatively broadest 

 in this species. The middle incisor is here 

 also complicated by a posterior notch and 

 an external groove. These modifications of the 

 external incisors of the kangaroos were first no- 

 ticed by Mr. Jourdan, and subgeneric distinc- 

 tions, with names often sufficiently unmeaning, 

 if not absurd,* have been subsequently ba?ed 

 upon them ; but such dental characters possess 

 neither sufficient constancy nor physiological 

 importance to justify such an application. 



M. Fr. Cuvier has proposed a binary divi- 

 sion of the genus Macropus, as here defined, 

 founded on the absence of permanent spurious 

 molars, and a supposed difference in the mode 

 of succession of the true molars in certain spe- 

 cies of Kangaroo, combined with modifications 

 of the muzzle or upper lip, and of the tail. 



The dental formula which I have assigned 

 to the genus Macropus is restricted in its ap- 

 plication by that naturalist to some small spe- 

 cies of Kangaroo, grouped together under the 

 term Halmaturus, originally applied by Illiger 

 to the Kangaroos generally .f The rest of the 

 Kangaroos, under the generic term Macro- 

 pus, are characterised by the following dental 



formula: Incisors, -; molars, . = 24. 



2 4 4 



The truth, however, is, that 

 both the Halmaturi and 

 Macropl of M. Fr. Cuvier 

 have their teeth developed 

 in precisely the same num- 

 ber and manner : they only 

 differ in the length of time 

 during which certain of these 

 teeth are retained.J In the 

 great Kangaroo, for exam- 

 ple, the permanent premolar 

 which succeeds the corres- 

 ponding deciduous one in 

 the vertical direction, is 

 pushed out of place and 

 shed by the time the last 



* E. g. Bettongia, Gray, Petrogale, Gray, which 

 signifies ' rock weasel.' 



t Prodromus Systematis Mammalium et Avium, 

 8vo. 1811. The dental character which this ex- 

 cellent naturalist gives, accurately expresses the 

 condition of the canine or laniary teeth, " La- 

 niarii aut nulli, aut superiores 2 ambigui, minuti, 

 in medio inter primores et molares collocati," p. 

 80 ; but there are never more than five molars in 

 place on each side of each jaw in the Kangaroo. 



J M. Fr. Cuvier was aware that a deciduous spu- 

 rious molar existed in the great Kangaroo and other 

 species of his siihgenus Macropus, but he believed 

 that it was peculiar to an early period of life, and 

 then existed only in a rudimental state, or " en 

 germe," and that instead of being displaced and 



