rHtEtic formations. 7 



tion of the coronal basis continuing to the fang from between two of the coronal tubercles 

 may be noted, in elucidation of the like character described and figured (in Letter) by Prof. 

 Phillips, F.R.S., in the tooth of Microlestes rhaticus, which is decisive against the alleged 

 resemblance and affinity signified by the generic name IL/psijn-i/mnopsis imposed upon 

 that tooth by Mr. Boyd Dawkins, F.R.S. (' Quart. Journ. Geol. Soc.,' vol. xx, p. 411.) 



PI. I, fig. 6, represents the crown of a lower molar, showing a well-defined neck and 

 the beginnings or bases of two equal fangs. The circumference of the crown describes 

 an irregular ellipse, the compression being from without inward. 



The Microlestian generic character of a smooth subdepressed central area, enclosed 

 by a thick tubercular wall, is well shown in this tooth. Tlie outer part of the wall 

 developes three tubercles, the divisions between which extend from their outer side upon 

 that surface of the base of the crown toward the fangs. The inner part of the wall {a) 

 developes four small tubercles, the anterior one, forming the antero-internal angle of the 

 crown, being the largest and most prominent. 



Were this tubercle to be broken off and the outer part of the tooth worn down 

 as far as the line b, the portion left of the working surface of the crown would offer 

 an instructive subject of comparison with the portion of the crown of the Microlestes 

 [Hypsiprymnopsis) rhceticus represented in PI. I, fig. 16, from a drawing of the original 

 kindly made for the author by Prof. Phillips, F.R.S. 



The subject of figs. 7 and 7 a, PI. I, is the crown, with the base of the two fangs, of 

 a lower molar; the working surface is more worn than in fig. 0. Consequently the 

 three outer tubercles of the coronal wall are represented by a plicate outline such as Avould 

 be presented by the premolar of Hypsijirymmis murinus, (ib., fig. 17) if it were worn down 

 to near the base of its enamel-foldings ; only the contrasted extreme shortness of the 

 crown in Microlestes brings the intervals of the tubercles nearer to the origins of the roots 

 of the tooth. The smaller tubercles of the inner side of the wall are almost obliterated 

 in this tooth, and the smooth central area is proportionally enlarged. A fourth lower 

 molar in an old Myrmecobius closely repeats this character. 



PI. I, figs. 8, 8 A, show the crown and beginning of the roots of a lower molar 

 with the trituberculate outer wall, and the summit of the anterior angular larger 

 tubercle of the inner wall. Fig. 8 a gives a view of the crown from the fore end or 

 side, showing the degree of compression from without inwards ; it also shows the hind- 

 most of the three outer tubercles, and the summits and intervals of the smaller tubercles 

 of the inner wall extending to the hind end of the crown. 



PI. I, fig. 9, represents the crown of a lower two-fanged molar with the working 

 surface but little worn, showing the three outer tubercles and the five smaller inner ones, 

 with the characteristic anterior large and prominent corner tubercle (broken off in fig. 16a). 



PI. I, fig. 10, represents the crown of a small molar viewed obliquely from the inner 

 side, showing the anterior larger tubercle followed by three smaller ones ; the three 

 larger tubercles of the outer wall are also brought into view. 



