236 HISTORICAL PALAZONTOLOGY. 
belonging to the genera Lxogyra and Gryphea (fig. 167). In 
the former of these are included Oysters with the beaks 
Fig. 166.—Ostrea Marshitz. Middle : 
and Lower Oolites. Fig. 167.—Gryphea incurva. 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 Exegvre is the Z. virgula of the Oxford 
Clay, and of the same horizon on the Continent; and the 
Gryphea 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. 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 4 Di- 
cerates,” applied to beds of 
the same age as the Coral- 
rag of Britain. The genus Dz- 
ceras belongs to the same fam- 
ily as the “ Thomy Clams” 
(Chama) of the present day— 
the shell being composed of 
=i ; nearly equally-sized valves, the 
Fig. 168.—Diceras arietina. Middle 1,,4ks 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- 
