17 



The main difference, as compared with Hypsiprymnus, lies in the 

 absence of the upper canines ; yet I have seen them present, but of 

 very small size, and concealed by the gum, in a small species of 

 Kangaroo (Macropus rufiventer, Ogilby.). This, however, is a rare 

 exception ; while the constant presence and conspicuous size of the 

 canines will always serve to distinguish the Potoroo from the Kan- 

 garoo, But besides this, there are other differences in the form and 

 proportions of certain teeth. 



. The upper incisors of the Macropi have their cutting margins on 

 the same line, the anterior ones not being produced beyond that line 

 as in the Hypsiprymni ; the third or exteruEd 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 

 obliquely forwards and inwards, into the substance of the tooth. 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 

 part of the tooth in front or behind it, vary more or less in everj"- 

 species of Macropus : there are two folds of enamel near the anterior 

 part of the tooth in Macr. major ; the posterior 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 

 with a posterior notch and an external groove. These modifications of 

 the external incisors have been pointed out in detail by M. Jourdan ; 

 and subgeneric distinctions have been subsequently based upon 

 them ; but they possess neither suSicient constancy nor physio- 

 logical consequence, to justify such an application. M. Fr. Cuvier 

 has proposed a binary division of the Kangaroos founded on the 

 absence of permanent spurious molares and a supposed difference 

 in the mode of succession of the permanent molares in the Kan- 

 garoos, 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 by that naturalist in its application to some small species 

 of Kangaroo, grouped together under the term Halmaturus, origin- 

 ally applied by Illiger to the Kangaroos generally. The rest of the 

 Kangaroos, under the generic term Macropus, are characterized by 



the following dental formula : — incisors 5 ; niol. j^^ : =s24. 



The truth, however, is, that both the Halmaturi and Macropi of Fr. 

 Cuvier, have their teeth developed in precisely the same number and 

 manner ; they only differ in the length of time during which certain 

 of them are retained. In the great Kangaroo, for example, the per- 

 manent spurious molar which succeeds the corresponding deciduous 

 one in the vertical direction, is pushed out of place and shed by the 

 •time the last true molar has cut the gum : the succeeding true molar 

 is soon afterwards extruded ; and I have seen a skull of an old Ma- 

 cropus major in the Museum at Leyden, in which the grinders were 

 reduced to two on each side of each jaw by this yielding of the an- 

 terior ones to the vis a tergo of their successors. 



