312 



THE GEOLOGIST. 



AMMONITES. AMMONITES. 



Emerici, RaspaiL Matheroni, d'Orb. 



Flexisulcatus, d*Orb. Nisus, d'Orb. 



Gargasensis, d'Orb. Pretiosus, d'Orb, 



Guettardi, RaspaiL Itaresculatus, d'Orb. 



Impressus, d'Orb, Royerianus, d'Orb. 



Inornatus, d*Orb, Striatisulcatus, d'Orb, 



Mar tinii, d'Orb. Strangulatus, d'Orb. 



We find^ then, that the Ammonites of the Neocomien for- 

 mation are divided into two distinct epochs, comprising dif- 

 ferent species, and that the decrease of the number, which I 

 have appropriated to each group, is apparent even in the 

 epochs themselves, as the upper strata contain considerably 

 fewer species than the lower. At present, I am not ac- 

 quainted with any species of the Neocomien formation^ 

 found simultaneously in the Jurassic formation, and none 

 extends beyond the clay a plicatules, composing the upper 

 portion of the Neocomien formation, to the lower strata of 

 the gault. I have not even found, in any instance, a place 

 where these strata are not perfectly distinct and separated 

 by sands ; they always contain their peculiar species ; thus,, 

 all the species I have named are characteristic of the Neoco- 

 mien formation. 



2nd. Group. — Gault or lower Greensand, 



The 42 species of Ammonites of the Gault or lower 

 Greensand, may also be divided into two series, the one 

 always appertaining to the low^er beds, the other to the 

 upper; but these divisions are much less strongly marked 

 than in the Neocomien formation, as we sometimes find the 

 two series confounded together, which does not appear to be 

 the case in the Neocomien formation. It is true that here the 

 thickness of the strata is never so great, and changes are 

 much more frequent. Whatever it may be, the following 

 are the species divided according to the upper and lower beds. 



