[Illustration: Fig. 84.--Portion of a mass of _Favosites Gothlandica_, of the natural size. Upper Silurian and Devonian of Europe and America. (Original.) Billings.] [Illustration: Fig. 85.--Fragment of _Favosites hemispherica_, of the natural size. Upper Silurian and Devonian of America. (After Billings.)] The commonest, and at the same time the largest, of these are the "honeycomb corals," forming the genus _Favosites_ (figs. 84, 85), which derive both their vernacular and their technical names from their great likeness to masses of petrified honeycomb. The most abundant species are _Favosites Gothlandica_ and _F. Hemispherica_, both here figured, which form masses sometimes not less than two or three feet in diameter. Whilst _Favosites_ has acquired a popular name by its honey-combed appearance, the resemblance of _Michelinia_ to a fossilised wasp's nest with the comb exposed is hardly less striking, and has earned for it a similar recognition from the non-scientific public. In addition to these, there are numerous branching or plant-like Tabulate Corals, often of the most graceful form, which are distinctive of the Devonian in all parts of the world. The _Echinoderms_ of the Devonian period call for little special notice. Many of the Devonian limestones are "crinoidal;" and the _Crinoids_ are the most abundant and widely-distributed representatives of their class in the deposits of this period. The _Cystideans_, with doubtful exceptions, have not been recognised in the Devonian; and their place is taken by the allied group of the "Pentremites," which will be further spoken of as occurring in the Carboniferous rocks. On the other hand, the Star-fishes, Brittle-stars, and Sea-urchins are all continued by types more or less closely allied to those of the preceding Upper Silurian. Of the remains of Ringed-worms (_Annelides_), the most numerous and the most interesting are the calcareous envelopes of some small tube-inhabiting species. No one who has visited the seaside can have failed to notice the little spiral tubes of the existing _Spirorbis_ growing attached to shells, or covering the fronds of the commoner Sea weeds (especially _Fucus serratus_). These tubes are inhabited by a small Annelide, and structures of a similar character occur not uncommonly from the Upper Silurian upwards. In the Devonian rocks, _Spirorbis_ is an extremely common fossil, growing in hundreds attached to the outer surface of corals and shells, and appearing in many specific forms (figs. 86 and 87); but almost all the known examples are of small size, and are liable to escape a cursory examination. [Illustration: Fig. 87.--a, _Spirobois omphalodes_, natural size and enlarged. Devonian, Europe and America; b, _Spirorbis Arkonensis_, of the natural size and enlarged; c, The same, with the tube twisted in the reverse direction. Devonian, America. (Onginal.)]