136 DE. B. A. BEXSLEY ON THE EVOLUTIOX 



conspicuous for its wealth of species, a condition which is doubtless due in a large 

 measure, as in the former genus, to the advent of a new and successful adaptation— in 

 tiie present case the formation of quadrate selenodont molars, in Fhalanger of sectorials. 

 JJ*etauroides, as pointed out by Thomas (ib88) is indistinguishable in dentition from 

 I'seudochirtis, and otherwise represents simply a volant form of the latter. Phascol- 

 arctus, althougli seemingly isolated by its great specialization, shows in all of its dental 

 characters a direct advance on Psetidochirus. 



Sequence of Molar T?aiterns. — Notwithstanding their wealth of numbers, the species 

 of Pseiidochirus are remarkably constant in tlieir molar characters, and the following 

 description of P. peregrinus will apjily almost equally to all. In P. peregrinus 

 (PI. 5. fig. 17) first, second, and third upper teeth are fully quadrate in shape, like 

 those of the advanced Phalangerimie and Peramelinye, so that they are not separated 

 internally by angular spaces. The functional cusps ai'e arranged in three longitudinal 

 rows, a condition which at first sight suggests that seen in the Peramelinse. On closer 

 examination, however, the teeth are seen to i:)0ssess a fourth (external) row of vestigial 

 cus])s representing the external styles of the third row in the Peramelinse and the poly- 

 protodont families in general. The predominant cusps of tlie crown are those of the first 

 and third roAVS, these representing, as in the Phalangerinae, the protocone, paracone, 

 hypocone, and metacone. The protocone is slightly predominant. The hyj)ocone is 

 complete. There is no disproportion in size between the paracone and metacone, and 

 no trace of a metacone-spur. Tiie main cusps are comparatively low and are crescentic 

 in section, each being concave in its outer face. The protocone and metacone are placed 

 slightly below the level of the outer cusps. The cusps of the second row are com- 

 paratively small, but are of the same crescentic character as the larger ones. They are 

 usually smaller than in the specimen here figured. They represent the subsidiary proto- 

 and metaconules, which are fairly common in the Placentals, but rare in Marsupials. The 

 vestigial external styles are two in number ; they ajjpear to represent b (or ah) and c of 

 polyprotodont forms. Their presence in the Phascolarctinae is of considerable interest, since 

 in all the remaining members of the diprotodont section they have completely disappeared. 

 Style c shows a slight tendency to be bifid, a condition which is partly natural and partly 

 the result of wear. In mastication the crowns of the lower teeth sweep transversely 

 across those of the upj)er in such a way that the triangular tips of the hypoconid and 

 entoconid pass through tlie triangular spaces separating the paracone and metacone. 

 Style c is placed directly in the line of passage, so that its tip becomes notched by those 

 of the hypoconid and entoconid. In unworn teeth of P. peregrinus and P. Cooki there 

 is seen a sUght indication of a cleavage of this element, showing that the mechanical 

 notching of its tip has produced a natural development in that direction. In other 

 species of Pseudochirus there is a slight variation in the external styles : they are 

 obsolete in P. DcMl, and almost so in P. Forbesi and in Peiauroides volam. 



Like those of the Phalangerinae, the upper molars decrease in size backwards. The 

 fourth tooth is reduced, although not to the same extent as in polyprotodont forms. 

 This element is in fact now undergoing a progressive evolution. In P. peregrinus it is 



