Thus of the true Oysters,
_Ostrea distorta_ is characteristic of the Purbeck series, where
it forms a bed twelve feet in thickness, known locally as the
"Cinder-bed;" _Ostrea expansa_ abounds in the Portland beds;
_Ostrea deltoidea_ is characteristic of the Kimmeridge clay;
_Ostrea gregaria_ predominates in the Coral-rag; _Ostrea acuminata_
characterises the small group of the Fuller's Earth; whilst the
plaited _Ostrea 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 _Gryphoea_ (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 _Exogyroe_ is the _E. Virgula_ of the Oxford
Clay, and of the same horizon on the Continent; and the _Gryphoea
incurva_ (fig. 167) is equally abundant in, and characteristic
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.
Pages:
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359