THE JURASSIC PERIOD. 



243 



Marshii (fig. 166) is a common shell in the Lower and Middle 

 Oolites. Besides the more typical Oysters, the Oolitic rocks 

 abound in examples of the singularly unsymmetrical forms 

 belonging to the genera Exogyra and Gryphaa (fig. 167). In 

 the former of these are included Oysters with the beaks 

 " reversed " that is to say, turned towards the hinder part of 

 the shell; whilst in the latter are Oysters in which the lower 

 valve of the shell is much the largest, and has a large incurved 

 beak, whilst the upper valve is small and concave. One of 

 the most characteristic Exogyra is the E. Virgula of the Oxford 

 Clay, and of the same horizon on the Continent; and the 

 Gryphaa incurva (fig. 167) is equally abundant in, and char- 

 acteristic of, the formation of the Lias. Lastly, we may 



Fig. 166. Ostrea Marshii. Middle 

 and Lower Oolites. 



Fig. 167. Gryphcea incurva. Lias. 



notice the extraordinary shells belonging to the genus Diceras 

 (fig. 168), which are exclusively confined to the Middle 



Oolites. In this formation in 

 the Alps they occur in such 

 abundance as to give rise to 

 the name of " Calcaire a Di- 

 cerates, " applied to beds of 

 the same age as the Coral- 

 rag of Britain. The genus Di- 

 ceras belongs to the same fam- 

 ily as the " Thorny Clams " 

 (Chama) of the present day 

 the shell being composed of 



nearly equally-sized valves, the 

 Fig. m.-Dicera^arietina. Middle beakg Qf whkh arg extremely 



prominent and twisted into a 



spiral. The shell was attached to some foreign body by the 

 l:eak of one of its valves. 



