Amongst the _Mollusca_, the Devonian rocks have yielded a great number of the remains of Sea-mosses (_Polyzoa_). Some of these belong to the ancient type _Ptilodictya_, which seems to disappear here, or to the allied _Clathropora_ (fig. 90), with its fenestrated and reticulated fronds. We meet also with the graceful and delicate stems of _Ceriopora_ (fig. 91). [Illustration: Fig. 90.--Fragment of _Clathropora intertexta_, of the natural size and enlarged. Devonian, Canada. (Original.)] [Illustration: Fig. 91.--Fragment of _Ceriopora Hamiltonensis_, of the natural size and enlarged. Devonian, Canada. (Original.)] [Illustration: Fig. 92.--Fragment of _Fenestella magnifica_, of the natural size and enlarged. Devonian, Canada. (Original.)] [Illustration: Fig. 93.--Fragment of _Retepora Phillipsi_, of the natural size and enlarged. Devonian, Canada. (Original.)] [Illustration: Fig. 94.--Fragment of _Fenestella cribrosa_, of the natural size and enlarged. Dovonian, Canada. (Original.)] The majority of the Devonian _Polyzoa_ belong, however, to the great and important Palæozoic group of the Lace-corals (_Fenestella_, figs. 92 and 94, _Retepora_, fig. 93, _Polypora_, and their allies). In all these forms there is a horny skeleton, of a fan-like or funnel-shaped form, which grew attached by its base to some foreign body. The frond consists of slightly-diverging or nearly parallel branches, which are either united by delicate cross-bars, or which bend alternately from side to side, and become directly united with one another at short intervals--in either case giving origin to numerous oval or oblong perforations, which communicate to the whole plant-like colony a characteristic netted and lace-like appearance. On one of its surfaces--sometimes the internal, sometimes the external--the frond carries a number of minute chambers or "cells," which are generally borne in rows on the branches, and of which each originally contained a minute animal. [Illustration: Fig. 95.--_Spirifera sculptilis_. Devonian, Canada. (After Billings.)] [Illustration: Fig. 96.--_Spirifera mucronata_. Devonian, America. (After Billings.)] [Illustration: Fig. 97.--_Atrypa reticularis_. Upper Silurian and Devonian of Europe and America. (After Billings.)]