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



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 Becklesii. In the great 

 development of the premolars, and the dwarfed size of the true molars, 

 there is in the fossil an analogy with Acrohata pygmcea, the "Opos- 

 sum-mouse," or *' Pigmy Flying Opossum" of New South Wales. 

 But the resemblance goes no further, the principal premolars in 

 Acrohata being much elevated and pointed in front, leaning to the 

 insectivorous type*, while they are uniformly compressed, grooved, 

 and serrated in Plagiaulax minor. 



This concludes what I have to offer in the shape of descriptive 

 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 directness of the indications : — 



1 . The compressed hatchet-shaped last premolar with the serru- 

 lated 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 fur- 

 rows 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 soli- 

 tary 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 inconsistent 

 with the marsupial indications. 



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

 at variance with a marsupial determination, since it has been asserted, 

 by an able authority, that, with the exception of the edentate " spe- 

 cies of marsupials, or those which are nearly edentate, like the 

 Tarsipes, and also excepting the Myrmecobius, all Marsupialia pos- 

 sess four true molars f." But the character is not absolute, for 

 all the Pigmy-Phalangers of the subgenus Dromicia, besides Acro- 

 hata, are admitted to have only three true molars J. In the Pur- 

 beck fossils the premolars are inordinately developed, while the 

 true molars are dwarfed and rudimentary in proportion. Where 

 such characters coexist with an exceedingly abbreviated alveolar 

 border, there is less reason for surprise in seeing two of the molars 



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



t Waterhouse, Nat. Hist, of Mammalia, vol. i. p. 8. This generalization was 

 previously stated by Professor Owen, in his memoir in the Zoological Transactions, 

 vol. ii. p. 333, of the 8th Jan. 1839. 



X Waterhouse, op. cit. pp. 307, 337. 



