PURBECK FORMATIONS. -39 



anterior margin of the coronoid process, as in the right ramus (fig. 5). A well-marked 

 linear groove {g) traverses the whole length of the inner surface of the portion of the 

 horizontal ramus preserved, about one third of the depth from the lower margin, ending 

 at the hind part of the symphysis. Its curve and commencement in advance of the 

 anterior boundary of the pterygoid fossa repeat the characters noted in the subject of 

 fig. 5, PL II. 



The left mandibular ramus (PI. II, fig. 7, nat. size, 7 A, magn. three diam. in tint), 

 wanting the ascending branch, has four well-preserved molars in place, the inner side 

 exposed. The mylohyoid groove {g) has the same commencement and course. The 

 better preserved, long, narrow, almost horizontal symphysis (r, s) shows that the slender 

 ramus gradually tapers to the obtusely pointed anterior end. 



The molars are closely arranged, and well display the anterior slender sharp-pointed 

 cusp (fig. 7, B, e) rather inclined forward ; next, a similar, but rather larger, longer, and 

 more nearly vertical, main cone (o), behind which is the basal cusp {s) : a very minute 

 accessory cusp could be detected at the inner and hinder part, near the base of the main 

 cone. 



There is space for one or two such molars, especially if the last one lost size, as in the 

 type jaw (fig. 5 a, m s), behind those in place ; the extent of the empty alveolar tract in 

 advance of the molars indicates teeth in number according to those shown in fig. 6. 



The four molars in place answer to those marked m 3—6, in PI. II, fig. 5 a. If the 

 proportion which those four teeth there occupy of the extent of alveolar tract from the 

 fore part of the coronoid to the fore end of the symphysis be compared with the corre- 

 sponding proportion in Amhlotherium soricinum (ib., fig. 1), their superiority, both 

 absolutely and in relative size to the jaw, in the present fossil, will be appreciated. But 

 the difference of "Structure, according to the taxonomic value assigned by Mammalogists to 

 the characters of true molars, is more than specific. The mylohoid groove [g) describes a 

 somewhat different curve from that in Amblotkerium. 



Species 2.- — Achyrouon pusili-us, Owen. Plate II, figs. 8, 8 a. 



The mandibular character last referred to has not more than specific vailue, if so much. 

 The curve, convex upward, of the hinder half of the mylohyoid groove (PI. II, fig. 8 a, g) 

 in the portion of jaw showing in the few molars preserved the generic coronal characters of 

 Acliyrodon is, however, associated with other differences. 



The specimen (ib., fig. 8, nat. size ; 8 a, magn. 3 diam.) is the hinder half of the 

 horizontal portion of a left mandibular ramus with the inner side exposed, showing four 

 molars in situ, which I determine to be the third to the sixth inclusive, as in fig. 7; there 

 are sockets of teeth, before and behind those in place, indicative of the same excess of 

 number as characterises the dentition of the preceding species of Achgrodon. I quote the 



