DIDELPHYS ? 73 



sent the same laterally compressed shape as in the fossil. 

 In addition to the perfect tooth, the fossil includes the 

 empty sockets of two other teeth, and the relative position 

 of these sockets places the Perameles out of the pale of 

 comparison. On the hypothesis that the present fossil 

 represents a species of Didelphys, the tooth in situ unques- 

 tionably corresponds with the second, or middle false molar, 

 right side, lower jaw. This is proved by the size and 

 position of the anterior alveolus. 



Had the tooth in situ been the one immediately pre- 

 ceding the true molars, the socket anterior to it should 

 have been at least of equal size, and in juxta-position to 

 the one containing the tooth. The anterior socket, how- 

 ever, is little more than half the size of the one in which 

 the tooth is lodged ; it is, also, separated from that socket 

 by an interspace, equal to that which separates the first 

 from the second false molar in the Didelphys Virginiana. 

 This is well shown in the inside view. In the placental 

 Mammalia, in which the first small false molar is similarly 

 separated by a diastema from the second, the first false 

 molar has only a single fang. In the present fossil, the 

 empty socket of the first false molar proves that the tooth 

 had two fangs, as in the marsupial Ferae and Insectivora. 

 There is nothing in the size or form of the socket, posterior 

 to the implanted tooth of the fossil, to forbid the supposi- 

 tion that it contained a false molar, resembling the one 

 in place ; had it been the socket of a true molar, then the 

 fossil could not have belonged to Didelphys, or to any other 

 known marsupial genus, because no known marsupial ani- 

 mal, which presents the posterior false molar of a similar 

 form and in like juxta-position with the true molars, as the 

 tooth in the present fossil, (on the supposition that it im- 

 mediately preceded the true molars,) has the next false 



