Upper Silurian Rocks. 95 



formed of Corals, Encrinites, Mollusca, and Trilobites, 

 Corals often predominating. The most characteris- 

 tic shell of the Aymestry limestone is Pentamerus 

 Knigktii. 



The grouping of fossils in the Upper Silurian rocks 

 is in general terms much the same as in the Lower series, 

 although new genera appear, but a very large proportion 

 of more than 700 Llandeilo and Caradoc species were 

 extinct in our area, only about 16^ per cent, being 

 common to the Lower and Upper series. The Corals, 

 which are in general not very numerous in British Lower 

 Silurian rocks, have increased to 82 species of 27 

 genera, of which 15 genera and about 65 species are 

 new. The Echinodermata (stone lilies) increase to 55 

 species, only 1 species of which, an Actinocrinus, is 

 common to Lower and Upper Silurian rocks. Several 

 new starfish appear, especially in the Upper Ludlow 

 rocks. There is one true Echinus (sea-urchin), Palce- 

 chinus. In Britain the Trilobites decrease to 30 genera 

 and about 130 species. Among the Hydrozoa the 

 Graptolites decrease to 3 species in Britain ; and there 

 are about 20 known species of Polyzoa. There are 21 

 genera and 126 species of Brachiopoda. Among these, 

 of the genus Atrypa there are 8 species. Athyris and 

 Obolus appear for the first time in lists of fossils. 

 Leptcena from 10, decreases to 6 species; Orthis from 

 58 to 21 ; while Rhynchonella increases from 12 to 16, 

 and Strophomena decreases from 27 to 15. Of the 

 genus Spirifera there are 3 species in the Lower Silurian 

 rocks, and 8 in the Upper. In all, 2 1 genera and about 

 126 species of Brachiopoda are known in the British 

 Upper Silurian strata, and 22 genera and 171 species 

 in the Lower. The Lamellibranchiate mollusca in- 

 crease from 17 to 18 genera, and from 71 to 87 species, 



