576 Geological Society: Anniversary AddresSf 18i2. 



good knowledge of positive geology ; and he is therefore peculiarly- 

 qualified to carry into effect his arduous enterprize of describing 

 the fossils of France in the order of the formations. He has com- 

 menced this vast undertaking by publishing during last year 139 

 plates, and upwards of 500 pages of text, on the Cephalopods of the 

 Chalk. It is only necessary to glance over the figures, to perceive 

 the care with which the different parts of the fossils are delineated. 

 I particularly recommend to your notice the new genera, named by 

 M. d'Orbigny '■'■Ancyloceras " and " Toxoceras," and which added 

 to the '■'■ Crioceras" recently introduced into the science, increase 

 that infinite variety of forms in which the great Ammonite family 

 expanded, previously to its total disappearance from the living 

 world. 



The Cephalopods, very rare in the upper beds of the chalk, oc- 

 cur in such prodigious quantities in the lower parts, and particularly 

 in the " Neocomian" group, as defined by continental geologists, 

 that they occupy all the Numbers hitJierto published of the ' Pale- 

 ontologie Fran9aise.' The Ammonites have been the object of espe- 

 cial study to M. d'Orbigny, and have led him to conclusions of the 

 highest interest, both zoological and geological. In the former 

 respect, his observations on the external characters of Ammonites, 

 and on the limits of their natural and accidental varieties ; of the 

 differences of sex, and particularly of age, are entirely original. 

 Following these remains through the period of their development, 

 he describes the transformations they undergo, and investigates the 

 laws of such changes. The chambers, or the internal characteristics 

 of Ammonites, the importance of which was long ago indicated by 

 Von Buch, have presented new features to M. d'Orbigny, which are 

 easily applied to the purposes of classification. I speak of the di- 

 stinction of the " selles et lobes en parlies paires etimpaires" accord- 

 ing as they are cloven at the extremity, or terminate in a conical 

 point. Combining this characteristic with that of the length of the 

 dorsal lobe, and with those afforded by the exterior ornaments of 

 the shell, the form of the back and mouth, between which there is 

 almost always a coincidence, M. d'Orbigny has made twenty-one 

 natural groups, of which eleven had been already established by 

 Von Buch, and ten are new. Of these twenty-one groups, seven are 

 peculiar to the Jurassic or Oolitic formations, ten to the cretaceous, 

 and four contain species common to both. 



M. d'Orbigny points out the modifications of species through 

 time and space, and shows the relation that exists between certain 

 forms and the beds which contain them. He recognizes three 

 new creations or replacements of the species of Ammonites during 

 the cretaceous period, and thus establishes, on zoological data, three 

 divisions of natural groups; — first the Neocomian*, second, the 



* We have to learn why the very well-defined British forraation, the Lower 

 Greensand, seems to be suppressed and merged by our opposite neighbours 

 in the " Systemc Ncocomien." Cannot the Lower Greensand be pre- 

 served and the Neocomian be considered as a marine equivalent of our 

 Wealden f 



