HISTORICAL PALEONTOLOGY. 



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

 the former of these are included Oysters with the beaks 



Fig. 166.-- Ostrea Marshii. Middle 

 and Lower Oolites. 



Fig. 167. Gryphcea incuiva. Lias. 



" 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 Exogyrce. is the E. -virgula of the Oxford 

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

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

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

 notice the extraordinary shells belonging to the genus Diceras 

 (fig. 1 68;, 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 

 beaks of which are extremely 

 prominent and twisted into a 

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

 beak of one of its valves. 



Amongst the Jurassic Univalves (Gasteropoda) there are 

 many examples of the ancient and long-lived Pleurotomaria ; 

 but on the whole the Univalves begin to have a modern 

 aspect. The round-mouthed (" holostomatous "), vegetable- 



Fig. 168. Diceras arietina. Middle 

 Oolite. 



