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Polyzoa were abundant; brachiopods were fairly common, though subordinate to the pelecypods; they were mostly rhynchonellids and terebratulids, which lived side by side with the ancient forms, like Crania and Discina.

This form, or a modification of it, we find to be characteristic of all pelecypods which are attached to a foreign object of support by the cementation of one valve.

The oyster may be taken as the type of the form adopted by attached pelecypods.

Another characteristic of attached pelecypods is camerated structure, which is most frequent and extensive in the thick attached valve.