[Illustration: Fig. 58.--A, _Monograptus priodon_, slightly enlarged. B, Fragment of the same viewed from behind. C, Fragment of the same viewed in front, showing the mouths of the cellules. D, Cross-section of the same. From the Wenlock Group (Coniston Flags of the North of England). (Original.)] The _Corals_ are very numerously represented in the Upper Silurian rocks some of the limestones (such as the Wenlock Limestone) being often largely composed of the skeletons of these animals. Almost all the known forms of this period belong to the two great divisions of the Rugose and Tabulate corals, the former being represented by species of _Zaphrentis, Omphyma, Cystiphyllum, Strombodes, Acervularia, Cyathophyllum_, &c.; whilst the latter belong principally to the genera _Favosites, Choetetes, Halysites, Syringopora, Heliolites_, and _Plasmopora_. Amongst the _Rugosa_, the first appearance of the great and important genus _Cyathophyllum_, so characteristic of the Palæozoic period, is to be noted; and amongst the _Tabulata_ we have similarly the first appearance, in force at any rate, of the widely-spread genus _Favosites_--the "Honeycomb-corals." The "Chain-corals" (_Halysites_), figured below (fig. 59), are also very common examples of the Tabulate corals during this period, though they occur likewise in the Lower Silurian. [Illustration: Fig. 59.--a, _Halysites catenularia_, small variety, of the natural size; b, Fragment of a large variety of the same, of the natural size; c, Fragment of limestone with the tubes of _Halysites agglomerata_, of the natural size; d, Vertical section of two tubes of the same, showing the tabulæ, enlarged. Niagara Limestone (Wenlock), Canada. (Original.)] [Illustration: Fig. 60.--Upper Silurian Star-fishes. 1, _Palasterina primoeva_, Lower Ludlow; 2, _Paloeaster Ruthveni_, Lower Ludlow; 3, _Paloeocoma Colvini_, Lower Ludlow. (After Salter.)] [Illustration: Fig. 61.--A, _Protaster Sedgwickii_, showing the disc and bases of the arms; B, Portion of an arm, greatly enlarged. Lower Ludlow. (After Salter.)]