PURBECK FORMATIONS. 37 



homologies of the last two premolars in Phascolestes. The molar series in Mr. Brodie's 

 specimen are, however, in too mutilated a state to support more than the conjecture 

 that it may belong to a Peralestian species, 



§ TX. Genus — Achyrodon,i Owen. Plate IT, figs. 5 — 8. 



Thp present genus is represented by four more or less mutilated mandibular rami of 

 small size (PI. II, figs. 5 — 8), retaining molar teeth (fig. 7 b) of the general tricuspid type 

 exemplified in Spalacotherium and Amblotherium, but with the externo-median or main 

 cusp (ib., o) sharp and slender, longer in proportion to its basal breadth, and with the 

 antei'o-internal cusp (ib., e) of similar shape and proportions, and of nearly equal height. 

 The resemblance of these cusps to needle-points suggested the generic name. The 

 postero-internal cusp (ib., s) retains the small proportions shown in fig. 1 b, PL II). The 

 main cusp (fig. 6 b, o), inclines slightly forward, and the front one (e) in a greater degree, 

 overlapping the hind cusp of the antecedent molar. 



The specimens exemplifying this curious needle-toothed type of Marsupial are less 

 perfect than some of those exemplifying the preceding genera ; but enough of the dentition 

 and alveoH may be recognised to show that the genus enters into the polyprotodont and 

 multimolar gi'oup of Enfoiiiophaffa. 



Species \. — Achyrodon nanus, Owen. Plate II, figs. 5, 5 a, 6, 6 a, 7, 7 a, and b. 



The first exemplar of this species is a right mandibular ramus mutilated at both ends, 

 but including eight molars of the generic type and two contiguous premolars (PI. II, fig. 5, 

 nut. size ; 5 A, magn. 3 diam.). The inner surface is exposed. In advance of a vertical 

 line dropped in front of the foremost of these premolars begins the symphysis (r), and as 

 much of the symphysial end of the ramus continues forward as forms the alveoli of two 

 teeth, probably, also, premolars. The last two premolars, m place {ps.pi, fig. 5 a), have 

 crowns higher than those of the succeeding molars : that of /; 4 is twice the height of that 

 of m 1, and is rather higher than that of jy 3. Both are conical, subcompressed, with apices 

 polished and a little blunted by use. A low cingulum crosses the base obliquely from 

 before downward and backward. Both are implanted by two fangs. The coronal pattern 

 of the molars is at once assumed. The long, slender, anterior cusp rises obliquely forward 

 from the fore end of the cingulum. The longer main cone contrasts by its sienderness 

 with that of the antecedent premolar: The low hind cusp is as well marked as in the 

 succeeding teeth. These very gradually increase in size to the third {m 3), the seventh 

 and eighth decreasing again. 



As much of the fore part of the coronoid process as is preserved rises straight and slopes 

 but little from the vertical : it resembles the corresponding part of the coronoid of 

 Amblotherium mustelula (PI. II, fig. 2), not the more sloping convexity of that in AmbL 



<iX"l>">'> .ic'us ; obnvs, dens. 



