P. BECKLESII AND P. MINOR. 411 



posterior lower margin and inflected angle; it shows three 

 premolars (p, m) in situ, with the empty sockets of the two 

 hack molars. Another specimen (PL XXXIII. fig. 7) fortu- 

 nately supplies these two back molars in situ. The lower 

 jaw (also of the right side) of the other species, Plagiaulax 

 minor (Plate XXXIV. fig. 2), is less perfect. It contains all 

 the teeth in situ, beautifully preserved, but it is mutilated 

 vertically behind the alveolar border ; the ascending ramus, 

 with all the proximal portion, being wanting. Besides the 

 two true molars, it contains four premolars instead of three, 

 as in the other species. 



Teeth. — Together, these specimens furnish nearly complete 

 evidence as to the characters of the lower jaw of the genus. 

 And, first in regard to the teeth, the dental formula is : — 



incis. 1 — 1 ; can. — ; prem. 3 — 3 ; mol. 2 — 2 = 12 in P. Becldesii. 



4 — 4 = 14 in P. minor. 



To save unnecessary technical details by reference to a 

 well-known existing genus, which will constitute in other 

 respects an important term of comparison, it may be stated 

 at once that the incisor of Plagiaulax Becldesii (PL XXXIII. 

 fig. 1, a, and PL XXXIV. fig. 1), in every particular of form, 

 namely, edge, point, and section, and in relative amount of 

 projection, bears a very close general resemblance to the 

 incisor of the marsupial Hypsiprymnus, or Kangaroo Eat, of 

 Australia; it differs chiefly in being, for the relative sizej 

 more robust and curved more abruptly upwards, in the fossil 

 animal. Its line of implantation in the socket is more ver- 

 . tical, and the alveolar sheath shorter and thicker. The 

 diasteme is exceedingly abbreviated, not exceeding a line in 

 length. 



The three premolars, in PI. BecMesii (Plate XXXIII. 

 fig. 1, p, m), are in the finest state of preservation, showing 

 the details of every minute character. They are limited to 

 three in all the specimens (see Plate XXXIII. figs. 4, 11, 12,' 

 and PL XXXIV. fig. 1). This is a point of some importance 

 to establish distinctly, as there are four of these teeth in the 

 other species. They form a closely adpressed and compact 

 series of very unequal size, diminishing rapidly in succes- 

 sion from the last to the most anterior. The last premolar 

 (PL XXXIII. fig. 5) presents a square oblong side, convex 

 from back to front, and sloping upwards and inwards to the 

 edge, which is finely serrulated, as in Hypsiprymnus, the ser- 

 ratures being caused by the terminations of about seven well- 

 marked parallel grooves, which descend upon the side, not 

 vertically as in Hypsiprymnus (Plate XXXIII. fig. 6, I), but' 



