650 Doubts respecting 



a slight examination, we cannot help remarking some resem- 

 blance between the Stonesfield fossil and the Did. Virginia- 

 na ; and indeed admitting that a first tooth is wanting in the 

 extremity of the jaw, and that more are missing in the inter- 

 val from the presumed canine to the first molar, — we have in 

 reality the very same number of the three kinds of teeth as in 

 the opossum, and even with some similitude in the manner of 

 their disposition; but all analogy entirely ceases when we 

 examine the form of the teeth separately. 



The incisors, instead of being very small, dilated and flat- 

 tened, or obtuse, and placed close together quite at the ex- 

 tremity of the jaw, as in the opossum, are strong, conical, dis- 

 tant, and almost all lateral. 



The canine, instead of projecting forward and'being curved, 

 compressed, very pointed and very strong, especially as com- 

 pared with the incisors, is on the contrary quite straight, co- 

 nical, and hardly longer than they are. 



Finally, the molars, instead of being arranged at very une- 

 qual distances, particularly the anterior ones, and sometimes 

 even remarkably unequal, as in the opossum, increase gradu- 

 ally from the first to the fourth, and then decrease as far as 

 the last. As to their form, which is almost uniform in the fos- 

 sil jaw, it has no similitude whatever either in the crown or 

 the roots, to the insectivorous arrangement displayed in so 

 great a degree by the four last molars of the opossum. 



But if the palmated lobate form of the molar teeth of Did. 

 Bucklandii does not in any respect resemble that which ex- 

 ists among the placental or marsupial quadrupeds, this is not 

 the case with regard to the other Stonesfield jaw, brought in- 

 to notice by M. Prevost. The resemblance with regard to 

 this is nearly complete, excepting only in number ; so that if it 

 be impossible to consider these two jaws as belonging to indi- 

 viduals of the same species, which I readily allow, it is diffi- 

 cult to look upon them as belonging to animals of different 

 classes ; — the structure of the dental system, which furnishes 

 the particular masticating organs of each class of Vertebrata 

 being here absolutely the same. 



Thus far we have been proving that the two fragments from 

 Stonesfield differ entirely from the jaws of the opossums, but 

 that they closely resemble each other, at least in the form of 

 the molars. It now remains for us to enquire if what we have 

 described in the dental system of the supposed Didelphis of 

 Stonesfield has any evident analogy with what we find in 

 any of the four classes of Vertebrata whose jaws are furnish- 

 ed with teeth ; that is to say, among the Mammalia, reptiles, 

 amphibious animals, and fishes. 



