THE UPPER SILURIAN PERIOD. 



121 



B 



numerous, but the variety of generic and specific types has 

 now become greatly reduced. All the branching and complex 

 forms of the Arenig, the twin-Grap- 

 tolites and Dicranograpti of the 

 Llandeilo, and the double-celled 

 Diplograpti and Climacograpti of 

 the Bala group, have now disap- 

 peared. In their place we have 

 the singular Retiolites,vtith its curi- 

 ously-reticulated skeleton ; and sev- 

 eral species of the single-celled genus 

 Monograptus, of which a character- 

 istic species (M. priodon) is here 

 figured. If we remove from this 

 group the plant-like Dictyonema, 

 which are still present, and which 

 survive into the Devonian, no 

 known species of Graptolite has 

 hitherto been detected in strata 

 higher in geological position than 

 the Ludlow. This, therefore, pre- 

 sents us with the first instance we 

 have as yet met with of the total 

 disappearance and extinction of a 

 great and important series of or- 

 ganic forms. 



The Corals are very numer- 

 ously represented in the Upper 

 Silurian rocks, some of the lime- 

 stones (such as the Wenlock Lime- 

 stone) 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 later belong principally to the genera Favosites, 

 Chcctctes, Halysites, Syringopora, Heliolites, and Plasntopora. 

 Amongst the Rugosa, the first appearance of the great and 

 important genus Cyathophyllum, so characteristic of the Palae- 

 ozoic 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 



Fig. 58. A, Monograptus prio- 

 don, slightly enlarged. B, Frag- 

 ment of the same viewed from be- 

 hind. 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.) 



