1857.] FALCONER PLAGIAULAX. 263 



dence as to the characters of the lower jaw of the genus. And, first, 

 in regard of the teeth, the dental formula is, — 



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



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 Becklesii (fig. 1, «, & fig. 14), in every parti- 

 cular 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-rats, of Australia ; it 

 differs chiefly in being, for the relative size, more robust in the fossil 

 animal, and curved more abruptly upwards. Its line of implantation 

 in the socket is more vertical, and the alveolar sheath shorter and 

 thicker. The diasteme is exceedingly abbreviated, not exceeding a 

 line in length. 



The three premolars, in PL Becklesii (fig. 1, p, m), are in the 

 finest state of preservation, showing the details of every minute cha- 

 racter. They are limited to three in all the specimens (see figs. 4, 

 11, 12, & 14). This is a point of some importance to establish di- 

 stinctly, as there are four of these teeth in the other species. They form 

 a closely adpressed and compact series of very unequal size, diminish- 

 ing rapidly in succession from the last to the most anterior. The last 

 premolar (k) presents a square oblong side, convex from back to front, 

 and sloping upwards and inwards to the edge, which is finely serru- 

 lated, as in Hypsiprymnus, the serratures being caused by the ter- 

 minations of about seven well-marked parallel grooves, which descend 

 upon the side, not vertically as in Hypsiprymnus (fig. 6, I), but 

 diagonally downwards and forwards, disappearing about the middle 

 of the crown-side, upon a smooth and discoidal surface. The enamel 

 below the four last grooves is inequal and raised into a well-marked 

 crenated step (fig. 1, k), which is exhibited on all the specimens. 

 The interior surface being adherent to the matrix, the characters 

 of the inner side of the last premolar are not shown by this spe- 

 cimen. But in two of the other specimens the inner surface of 

 all the premolars is free and seen to be furrowed by diagonal grooves, 

 exactly like the outer, the principal difference being that the inner 

 side is more flattened, and the enamel smoother than on the outer. 

 The two sides, therefore, slope from the base upwards to a sharp or 

 thin edge, which is serrulated, agreeing in every respect, except the 

 inclination of the grooves, with the corresponding tooth of Hypsi- 

 prymnus. The tooth is implanted by two distinct fangs. The 

 penultimate premolar is somewhat spathulate in outline, the lateral 

 surface of the crown is convex in the longitudinal direction, and 

 slopes inwards to the apex, which is diagonally grooved like the last 

 premolar, the grooves being fewer in number. It is inserted by two 

 inequal fangs, the posterior of which is barely visible ; in size it cor- 

 responds with one fang division of the last premolar. The antepen- 

 ultimate or anterior premolar is greatly reduced in all its dimensions, 



VOL. XIII. PART I. U 



