P. BECKLESII AND P. MINOR. 413 



towards the diasteme froin the anterior 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 hi the more perfect jaw 

 (Plate XXXIII. figs. 1 and 4, m and i). But they are shown 

 in situ, in the most perfect preservation, in the jaw of 

 Placjiaulax minor (Plate XXXIY. fig. 2, m). It is clearly 

 apparent from the relation of the second tooth to the muti- 

 lated base of the anterior margin of the ascending ramus (6) 

 in the latter, and from the empty sockets of the fallen teeth 

 in the alveolar rim of the perfect specimen (Plate XXXIII. 

 fig. 4, i) of PL 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 determina- 

 tion, 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 and 4 of 

 Plate XXXIII. shows the whole of the teeth of PL BecMesii 

 in nearly as perfect preservation as they are in the specimen 

 of PL minor (PL XXXIV. fig. 2). 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 Placjiaulax 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 

 crown is little more than one-third of that of it. They are 

 situated both horizontally and vertically in different planes 

 from the premolars ; a perpendicular from the acute serrated 

 edge of the latter would coincide with the line of the inner 

 row of the crown-lobes of the true molars, to be described in 

 the sequel, as occurs in the recent Hypsiprymnus ; and the 

 outline of the coronal surface of the whole molar series would 

 be included within a curve, of which the last premolar formed 

 the most salient part, with a considerable descent on either 

 side of it. 



The crown of the first true molar in Placjiaulax minor 

 (Plate XXXIV. figs. 3 and 4), which I take first as the more 

 perfect, is of a broad oblong form. Its inner or axial margin 



