228 Geological Society : — Anniversary of 1839. 



it becomes our business to define those terms, so that they shall 

 be capable of expressing truths which include in their circuit the 

 past as well as the present animal and vegetable population of the 

 world. 



An example of this process has occupied a large portion of our 

 attention during the past year. It appeared to be a proposition 

 universally true, that the oldest strata of the earth's surface con- 

 tained cold-blooded animals only ; and that creatures of the class 

 mammalia only began to exist on the surface after the chalk stra- 

 ta had been deposited and elevated. And when, to a rule of this 

 tempting generality, a seeming exception was brought under our 

 notice, it became proper to examine, whether the anatomical line, 

 Avliich enables us to separate hot-blooded from cold-blooded ani- 

 mals, had really been rightly drawn ; and whether, by rectifying 

 the supposed characteristic distinction, the exception might not 

 be eliminated. The exception on which this very instructive 

 point was tried consisted in a few jaw-bones of a fossil animal, 

 which, though occurring in the Stonesfield slate near Oxford, a 

 bed belonging to the oolite formation, had been referred by Cu- 

 vier to the genus Didelphys, and thus placed among marsupial 

 mammals. In August last, M. de Blainville stated to the Acad- 

 emy of Sciences of Paris, his reasons for doubting the justice of 

 of the place thus assigned to the fossil animal. Founding his 

 views principally upon the number and nature of the teeth of 

 the fossil, he ssserted that the animal, if a mammal, must come 

 nearest the phocas ; but he rather inclined to believe it a saurian 

 reptile ; following, as he conceived, the analogies offered by a 

 supposed fossil saurian described by Dr. Harlan of Philadelphia, 

 and termed by him Basilosaurus. M. Valenciennes, on the other 

 hand, asserted the propriety of the place assigned by Cuvier to 

 the fossil animal, although he made it a new genus ; and gave 

 to the species the name Thylacotheriuvn Prevostii. The contro- 

 versy at Paris had its interest augmented when Dr. Buckland in 

 September carried thither the specimens in question. From 

 Paris the controversy was transferred hither in November, and 

 principally occupied our attention at our meetings till the middle 

 of January. 



One advantage. resulting from the ample discussion to which 

 the question has thus been subjected, has been, that even those 

 of us who were previously ignorant of the marks by which zoo- 



