RH^TIC FORMATIONS. 9 



was depressed, flat and smooth in the centre. This coronal border (fig. 16, a) was 

 disposed in four, probably five, tubercles ; on the latter alternative the fifth is broken 

 away, most likely as being the largest and most prominent. The attrition of the masti- 

 catory tubercles of the preserved border of the crown of this little molar has exposed 

 the dentine. " The crown of the tooth," writes Prof. Phillips, " is obliquely worn, and 

 on the worn surface are little cusps, as in my figure, also worn, but a little projecting at 

 the edges, as at fig. 16 a, as if they were formed of enamel and dentine within the general 

 border of enamel." ^ 



Now, this is precisely the appearance which the similarly worn multituberculate 

 border of certain molars of Microlestes present. The lofty compressed trenchant pre- 

 molars of Hypsiprymnus (fig. 17) lose all trace of the grooves and ridges at the basal part 

 of the crown in proximity with the fangs. The bulging basal part of the premolar is 

 covered by a smooth polished coat of enamel in both Plagiaulax (PI. IV, figs. 9, 10), 

 Hypsiprymnus (PI. I, fig. 17), and Bettongia, (ib., fig. 18). 



In Hypsiprymnus it is very true that the ' length ' or vertical diameter of the crown 

 of the tooth is great compared with the breadth or transverse diameter; but in this 

 ' main characteristic ' the Rhsetic tooth, like those of Microlestes and the true molars of 

 Plagiaulax, difiers widely from the premolar of Hypsiprymnus. The oldest Rat-Kangaroo 

 shows no such wearing down of the crown as must have happened to the rhsetic tooth 

 (fig. 16), if it ever possessed one as lofty as that of the hypsiprymnal premolar (figs. 17, 18). 

 " The implantation by two fangs [is] seen alike in both" the Rhsetic tooth and the premolar 

 of Hypsiprym^iui" : add, also, in Microlestes (lower molars). Only there is a vagueness of 

 meaning in Mr. Boyd Dawkins' phrase " seen alike ;" for though, doubtless, the Rhsetic 

 molar and the hypsiprymnal premolar, with divers other teeth, are each implanted " by 

 two fangs," those fangs are not " alike " in the teeth compared ; and where the field of 

 comparison is so restricted — where so few characters can be securely got out of the worn 

 and broken fossil denticle in the slab of hard marlstone — it is satisfactory to have anything 

 so plain and conspicuous to reason from as the two fangs so unlike in form and propor- 

 tions as are those in Hypsiprymnus (fig. 17) and in the so-called Hypsiprymnopsis 

 (fig. 16 a). The proportions of the fangs in the Rhsetic tooth, supposing them to be 

 the only two it possessed, are much more alike in it and in the lower molar of Microlestes 

 (PL 1, figs. 7 a, 14, 15). 



The hind fang in Hypsiprymnus is more than twice the breadth, antero-posteriorly, of 

 the front fang ; it is subcompressed like the crown it helps to support, and in Hypsi- 

 prymnus minor (PL I, fig. 1 7) its proportion of the crown is such as to include aU the four 

 vertical ridges and grooves that impress the upper half of the crown. 



Mr. Boyd Dawkins figures the anterior (broken) root of the Rhsetic denticle as 

 stouter, i.e. antero-posteriorly broader, than the posterior root, and describes the 



' Letter, ut supra. 2 g Dawkins, op. cit., p. 410. 



2 



