264 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 11, 



being hardly one-fourth the size of the tooth immediately behind it ; 

 in form it exhibits more the ordinary appearance of an incisor. 



In the other species, Plagiaulax minor (fig. 15), which is very 

 considerably smaller, the incisor {a) presents a corresponding general 

 form, but it is more elongated, less robust, and is not so much 

 curved upwards. A portion of the point has been broken off in the 

 specimen, and it is seen by the impression («') that the inner side 

 near the apex was hollowed out in a longitudinal depression. The 

 premolars, in number four, are higher in proportion to the depth of 

 the jaw than in the other species. The last one is similar in form 

 and grooving to the corresponding tooth in Plagiaulax Becklesiiy 

 but exhibits a slight difference in the inequality of the enamel-surface 

 below the basal terminations of the grooves. In front of it there 

 are two spathulate premolars, i. e. the antepenultimate and penulti- 

 mate, both diagonally grooved near the apex ; and at the base of the 

 antepenultimate, but pressed somewhat inwards, there is a very mi- 

 nute anterior or first premolar. The basal enamel- surface bulges 

 out over the fangs in these teeth in a rounded angle which points 

 downwards. Regarded as a series, they decrease in size very rapidly 

 from the last to the foremost. The sharp edge of the crowns of the 

 three anterior teeth slopes down towards the diasteme from the an- 

 terior margin of the last premolar ; while that of the latter slopes in 

 a reverse manner downwards and backwards towards the true molars, 

 the anticlinal planes meeting at an obtuse angle. 



The true molars in both species were limited to two, the sockets 

 of which alone remain in the more perfect jaw (figs. I &4,m and i). 

 But they are shown in situ, in the most perfect preservation, in the 

 jaw of Plagiaulax minor (fig. 1.5, m). It is clearly apparent from 

 the relation of the second tooth to the mutilated base of the anterior 

 margin of the ascending ramus (b) in the latter, and from the empty 

 sockets of the fallen teeth in the alveolar rim of the perfect specimen 

 (fig. 4, i) of PI. BecMesii, that the true molars in the lower jaws of 

 both species did not exceed two, a very unexpected and reduced 

 number, to occur in forms otherwise inferred to be marsupial, and 

 therefore demanding rigorous determination, there being no other 

 corresponding instance known within the whole range of this sub- 

 class, fossil or recent. 



Fortunately the specimen represented by figs. 1 & 4 shows the 

 whole of the teeth of PL BecMesii in nearly as perfect preservation 

 as they are in the specimen of PI. minor (fig. 15). It consists of a 

 right lower jaw in two continuous fragments, presenting the two last 

 molars of an adult animal, well worn and in situ. The anterior edge 

 of the ascending ramus is entire, forming a well-defined boundary 

 to the alveolar border, and so closely contiguous to the second and 

 last true molar, which it partly overlaps, that it is manifest, there 

 could not have been more than two of these teeth in the jaw. The 

 true molars are not only reduced in number in Plagiaulax minor^ 

 but they are also dwarfed in size, and comparatively insignificant 

 in contrast with the last premolar, the united length of the two 

 being less than that of the latter, while the vertical height of their 



