THE UPPER SILURIAN PERIOD. 



119 



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, with its curi- 

 ously-reticulated skeleton; and seve- 

 ral species of the single-celled genus 

 Moiiograptus^ of which a character- 

 istic species i^M. priodon) is here 

 figured. If we remove from this 

 group the plant-like Dictyonemcs, 

 which are still present, and which 

 survive into the Devonian, no 

 known sj^ecies 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 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, 

 Cystipkylhim, Strombodes, Acervularia, Cyathophyllum, &c. ; 

 whilst the latter belong principally to the genera Favosites, 

 Chcetefes, Halysites, Syringopoi^a^ Heliolites^ and Plasinopora. 

 Amongst the Rugosa, the first appearance of the great and 

 important genus Cyathophylluni, so characteristic of the Palae- 

 ozoic period, is to be noted ; and amongst the Tabulata 

 w^e have similarly the first appearance, in force at any rate, 

 of the widely-spread genus Favosites — the " Honeycomb- 

 corals." The '' Chain-corals" {Haly sites), figured below (fig. 59), 

 are also very common examples of the Tabulate corals during 

 this period, though they occur likewise in the Lower Silurian, 



Fig. 58. — A, Monograptus prio- 

 don, slightly enlarged. B, Frag- 

 ment 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 Flagsof the North 

 of England). (Original.) 



