P. BECKLESII AND P. MINOR. 415 



centre of the crown is occupied by a hollow — smooth, and, 

 as it were, scooped out and depressed considerably below the 

 raised margins. A well-marked chasm intervenes between 

 the marginal edges, both in front and behind, the latter be- 

 ing narrower and less pronounced, so that when the tooth is 

 viewed endwise, the opposite rows are seen apart somewhat 

 as hi the tooth of Microlestes. The crenation of the outer 

 margin into a row of tubercles is more decided in Microlestes l 

 (PI. XXXIV. fig. 7), the crown is narrower and more elon- 

 gated in proportion, and the opposed rows are more approxi- 

 mated. None of the raised points of these teeth in Plagiauiax 

 minor show any considerable marks of abrasion, nor exposure 

 of the ivory ; the white spots in the figures represent adherent 

 specks of matrix, and not depressed discs of wear ; the apices 

 of the outer crenations alone are a little worn in the last 

 molar. The animal from which the fossil was derived is in- 

 ferred, from the intact condition of the molar teeth, to have 

 been a young adult, as these teeth have been found well worn 

 in one specimen of the other species. 



In the jaw of Plagiauiax BecMesii (PL XXXIII. figs. 1 

 and 4), there are three small pits on the alveolar border, 

 behind the last premolar. The anterior two, seen upon the 

 fragment fig. 1, a, b, are closely approximated, with a thin 

 plate intervening, indicating that they are the sockets of the 

 two-fanged first molar. One of them is shown in the niche 

 at the apex of the vertical sections, PI. XXXIII. figs. 2 and 

 3,/, g. The last pit, fig. 4, *, is larger, square, and undivided, 

 but shows obsolete marks of a mesial constriction, indicating 

 that the second or last molar had fangs converging in a com- 

 mon socket. The rim of this socket is distinctly defined, and 

 the inner wall raised into a prominent gibbosity which leaves 

 a deep impression on the matrix. This gibbous point corres- 

 ponds with what occurs in the lower jaw of the recent Hypsip- 

 rymnus Gaimarcli, with which it was compared, and is commonly 

 seen on the alveolar wall of the last molar of other animals. 



In the second specimen of Plag. BecMesii, PI. XXXIII. 

 figs. 7-10, already referred to, the two true molars of an 

 adult, and, judging by their wear, well-aged animal, are seen 

 in situ. They present the same general characters as the 

 corresponding teeth of Plag. minor (PL XXXIV. fig. 2), but 

 appear to be proportionally larger, in comparison with the 

 premolars. The crown of the first molar bore, as in the 

 other species, three principal points ; two to the inner row ; 



1 I am indebted to Sir Charles Lyell 

 for the only available means of institut- 

 ing a comparison of these mammalian 

 teeth from Purbeck with those of the 



Microlestes of Plioninger. Besides se- 



veral duplicate casts, he had very careful 

 and highly finished drawings made of 

 the teeth during his last visit to Stutt- 

 gart, the whole of which have been 

 placed at my disposal. 



