422 PLAGIAULAX. 



margin. The incisor (a) is projected with a less siidden 

 curve upwards, and its sheath is longer than in PI. BecMesii. 

 The premolars are also larger in proportion to the height of 

 the jaw than in that species. Unluckily the whole of the 

 ascending ramus is wanting, and with it are lost the signi- 

 ficant characters yielded by the form of the condyle, coronoid 

 process, and posterior angle. At the fractured posterior end 

 (b), a small portion remains of the external oblique ridge 

 which rises into the anterior border of the coronoid. A well- 

 marked wide depression is seen on the posterior part of the 

 horizontal ramus under the true molars, corresponding with 

 that shown on the jaw of Plagiaulax BecMesii. In the great 

 development of the premolars, and the dwarfed size of the 

 true molars, there is in the fossil an analogy with Aerobata 

 pygrncea, the ' Opossum-mouse,' or ' Pigmy Flying Opossum ' 

 of New South Wales. But the resemblance goes no further, 

 the principal premolars in Aerobata being much elevated and 

 pointed hi front, leaning to the insectivorous type, 1 while 

 they are uniformly compressed, grooved, and serrated in 

 Plagiaulax minor. 



This concludes what I have to offer in the shape of descrip- 

 tive details. I shall now proceed to consider what may be 

 legitimately inferred from them respecting the nature and 

 affinities of the fossils. That the genus was mammal admits 

 of no question ; that it was a marsupial is inferred for the 

 following reasons, which are given in the order of the direct- 

 ness of the indications : — 



1. The compressed hatchet-shaped last premolar with the 

 serrulated edge and parallel grooving. These characters are 

 confined, among all known mammals, to the marsupial genus 

 Hypsiprymnus ; the correspondence in grooving is so exact 

 that the number of furrows is the same in the fossils and in 

 the recent species with which they were compared, namely, 

 seven; the difference, that they are diagonal in the former 

 and vertical in the latter, being trivial and not typical. 



2. The agreement in form, relative size, and direction of 

 the solitary incisor in the fossil rami, with that of the recent 

 Hypsiprymni. 



3. The indication of the raised and inflected fold of the 

 posterior inner and lower margin of the ramus. 



4. The form and characters of the symphysial suture. 



5. The absence of any character in the jaw or teeth incon- 

 sistent with the niarsivpial indications. 



The presence of only two true molars might seem, at first 

 sight, at variance with a marsupial determination, since it 



1 Waterhouse, Nat. Hist, of Mammalia, vol. i. p. 338. 



