80 THE LIAS AMMONITES. 



In the Department of the Cher, at Saint-Amand this zone is represented by Aeg. 

 Jamesoni •=zRegnardi, d'Orh., Pk^ll. Zoscomdi, Sow., Aeg. Valdani, d'Orb., Aeg. Mauge- 

 nesti, d'Orb., Amal. Ibex, Quenst., Aeg. Centaurus, d'Orb., Aeg. brevispina, d'Orb. In the 

 Department of the Aveyron there is a considerable thickness of Middle Lias composed of 

 limestones and marls. The limestones form the base, the " Zone a Lgt. Jimbriatum,^^ and 

 the marls occupy the upper portion, the " Zone a Amal. margaritatus." Specimens of 

 Aeg. Jamesoni have been found in the region, so that the fossils of this zone have not 

 been separated from i\iQ fimbriatum-hQA.^, as in other localities. In the Department of the 

 Rhone M. Dumortier has studied the fossils of the Middle Lias with great care, and has 

 shown that this division of the formation attains a considerable development in the 

 Basin of the Rhone. He observes, " Considered in relation to its fossils the Middle 

 Lias may be regarded as the reign of Belemnites, for in no other division of the 

 Secondary strata do we recognise the remains of these Cephalopods accumulated in such 

 considerable numbers and appertaining to so many species. The Ammonites also 

 furnish always in lower portions of the stage a very remarkable contingent of large 

 species, and all characteristic of special horizons. The Acephala, Gastropods, and 

 Brachiopods also give a considerable number of species." This author divides the 

 Middle Lias of the Basin of the Rhone into two great zones, very unequal otherwise in 

 their thickness. 



The lower is much more developed vertically, and comprises an assemblage of marly 

 limestones and marls, having a total thickness often exceeding eighty metres = 260 feet, 

 and forming the zone of Belemnites clavatus. The upper zone is very clearly separated 

 from the lower by its mineralogical composition and by its fossils ; and is characterised 

 by the presence of Pecten aquivalvis. The limestones which compose it are throughout 

 very hard, of various colours, very badly stratified, and form a thickness which, in the 

 centre of the basin, does not appear to exceed from six to eight metres = 20 to 26 feet. 



Table op the Middle Lias in the Basin of the Rhone. 



Upper Lias. 



Zone of Pecten aquivalvis. 

 Petrology. Thickness. Fossils. 



Calcareous " lumachelle," yellowish or reddish 



Heavy limestone, sublamellar, very hard, yel- 

 lowish-brown, with large, ferruginous, 

 oolitic grains 



Metres. 

 2 to 3 



2 to 5 



Limea acuticosta, Goldfuss. 

 Ostrea sportella, Dumort. 



