[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 Palaeozoic 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.)]

The Brachiopods still continue to be represented in great force through all the Devonian deposits, though not occurring in the true Old Red Sandstone.  Besides such old types as Orthis, Strophomena, Lingula, Athyris, and Rhynchonella, we find some entirely new ones; whilst various types which only commenced their existence in the Upper Silurian, now undergo a great expansion and development.  This last is especially the case with the two families of the Spiriferidoe and the Produclidoe.  The Spirifers, in particular, are especially characteristic of the Devonian, both in the Old and New Worlds—­some of the most typical forms, such as Spirifera mucronata (fig. 96), having the shell “winged,” or with the lateral angles prolonged to such an extent as to have earned for them the popular name of “fossil-butterflies.”  The closely-allied Spirifera disjunda occurs