310 



THE GEOLOGIST. 



2nd. that they have already diminished nearly one half in 

 point of number, in the Gault; 3rd. that they undergo a 

 fresh diminution of one half of the number found in the 

 Gault, at the epoch of the chloritic chalk, and have totally 

 disappeared with the middle beds of the third epoch, when 

 not a single species remains. The race of Ammonites are 

 no longer found in the upper strata of the Cretaceous for- 

 mation, the white chalks or the upper chalks. The Ammo- 

 nites have then progressively diminished in number, from 

 the lower to the upper groups of the cretaceous formation. 



I shall now proceed to study the species of each group 

 comparatively, and notice the species which are peculiar or 

 common to each. 



1st. Group, Neocomien Formation, 



The )b species of Ammonites of the Neocomien formation 

 do not all belong to the same geological horizon; and I 

 have little doubt that we shall be able to divide them into 

 distinct stages, by comparing them with the well-defined 

 strata which we observe in the department of the Haute- 

 Marne; but until the analogy of these strata with those 

 which contain Ammonites in Provence and the Alps shall 

 have been determined by attentive observation, I only deem 

 it proper to divide them into the groups of the upper and 

 lower Neocomien. 



Species of Ammonites peculiar to the Lower Neocomien. 



AMMONITES. AMMONITES. 



Angulicostatus^ d^Orb. Cassida^ RaspaiL 



Asperrhnus, d'Orb. Castellanensis, d'Orb. 



^stierianus, d'Orb. Clypeiformis, d'Orb, 



BidichotamuSy Lequerie. Compressissimus, d^Orb, 



^Calypso, d'Orb.{l) Cryptoceras, d'Orb. 



Carter on, d^Orb. Cultratus, d'Orb. 



(1) The species preceded by an asterisk are those to which I have some 

 doubts of their belonging to the Neocomien formation. 



