236 



HISTORICAL PALEONTOLOGY. 



belonging to the genera Exogy?'a and Gryphcea (fig. 167), In 

 the former of these are included Oysters with the beaks 



Fig. 166. -^Ostrea MarsJiii. 

 and Lower Oolites. 



Middle 



Fig. ib^.— GryphcBainctova. 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, virgiila of the Oxford 

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

 GryphcEd inciirva (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. 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 " 

 iyChama) 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 Pleuroto7Jiaria ; 

 but on the whole the Univalves begin to have a modern 

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



168. — Diceras a7-ieti7ta. Middle 

 Oolite. 



