324 



THE GEOLOGIST. 



duced to a single species, which is even of a different cha- 

 racter, in the chloritic chalk. This group is therefore 

 more particularly proper to the gault. 



Tuberculati, d^Orb. They are unknown in the Neocomien, 

 at their maximum in the lower gault, and reduced to a 

 single species in the chloritic chalk. 



Clypeiformi, d'Orb. are found in the three divisions of 

 the Cretaceous formation. 



PnlchelU, d^Orb. belong to the Neocomien and gault. 



Rhotomagenses, d'Orb. Peculiar to the chloritic chalk, the 

 only species of the gault having characters different from the 

 rest. 



Bentati^ Buch. Only proper to the Neocomien and gault, 

 unknown in the chloritic chalk ; they are particularly nu- 

 merous in the gault. 



FleJLUOsi, Buch. Are only found in the lower Neocomien. 



Compressi, d^Orb. Few in number in the Neocomien and 

 gault ; they are at their maximum in the chloritic chalk. 



Angulicostati, d'Orb. Peculiar to the Neocomien and 

 gault. 



Ligati, d'Orb. The species are very numerous, especially in 

 the Neocomien and gault ; they are few in the chloritic chalk. 



We thus find, that, out of the Twenty-one groups esta- 

 blished from their zoological characters, 



1st. Seven are peculiar to the Oolitic or Jurassic forma- 

 tions. 



27id. Ten to the Cretaceous formation. 

 Srd. Four contain species belonging to both formations. 

 I shall make a few observations before resuming the sub- 

 ject. 



Out of the ten groups peculiar to the Cretaceous forma- 

 tion, four (the Cristati, Clypeiformi, Compressi and Ligati), 

 are found in all three divisions, but the first is most common 

 in the gault, the third in the chloritic chalk ; the last in the 

 two lower divisions. Three groups (the Pnlchelli, Deniati, 



