PURBECK FORMATIONS. 81 



Plagiaulax Becklesii (?).i .Plate IV, figs. 12, 12 a, 12 b. 



The specimen (PI. IV, fig. 12, nat. size, 12 a, magn. 3 diam., b, grinding surface of 

 molars, magn. 3 diam.) referred to this species ^ is a fragment of the right mandibular 

 ramus with the beginning of the fore part of the base of the corouoid process (fig. 12 

 A, c) : it includes two tubercular molars in situ, with the inner side exposed, and the 

 impression of the obliquely ridged trenchant crown of a premolar (ib. p 4) in relative 

 superiority of size to the tuberculars corresponding with that indicated by the tooth in 

 place {p 4) and the molar alveoli in the preceding specimen (ib. fig. 10 a). To the 

 species represented by that specimen I should refer, without doubt, the present fragment, 

 but that the distance from the hind margin of the last premolar to the back of the socket 

 of the hind molar is relatively greater in the present specimen ; yet not to such a degree 

 as to forbid the reference, with some indication of doubt, of the present specimen to the 

 Plagimdax Becklesii. Accepting this determination, we can complete herewith the 

 description of the mandibular dentition of that species. 



The penultimate molar (fig. 12 a and b, m 1) presents a low oblong crown, the fore- 

 and-aft extent exceeding by one fourth the transverse diameter. The outer half is worn 

 lower than the inner half of the grinding surface, and the outer side of the higher inner 

 border is worn vertically smooth to a shallow groove, traversing the working surface 

 from before backward, and dividing the inner from the outer part of that surface. 



The inner surface, swelling out above the two roots of the tooth, presents a low obtuse 

 conical rising anteriorly, with an anterior basal low ridge. The posterior half of the 

 inner side of the crown is lower, and presents two small risings, divided by a shallow 

 longitudinal indent. The outer and lower worn-down side of the crown shows two or 

 three feeble risings. 



In the last molar (ib. vi 2) the inner half of the crown is higher than the outer one, 

 toward which it also presents a smooth, straight, sheer, or vertically worn, surface. But it 

 is on a lower level than m 1, the enamel being detached or broken away, which gives the 

 appearance of the tooth having been worn down more nearly to the fangs than is the 

 penultimate molar. This, however, is the result of accident. Sufficient of the inner half 

 of the molar remains to show the vertical surface presented to the lower outer half; and 

 such surface, indicative of vertical abrasion by an upper molar, would be, probably, as 

 high as in the penultimate molar (?« 1), had the enamel of the inner half of the crown 

 been preserved in m % The outer half of the crown retains its enamel covering ; has a 

 convex outline from before backward, with an almost flat, slightly concave, working 

 surface. The outer and fore part of the base of the coronoid is external to the posterior 

 two thirds of the last molar. 



' By Dr. Falconer, loc. cit., p. 279, figs. 7 — 10. 



II 



