THE UPPER SILURIAN PERIOD. 



127 



and the other compound. The feelers are converted into 

 pincers, whilst the last pair of limbs have their bases covered 

 with spiny teeth so as to act as jaws, and are flattened and 

 widened out towards their extremities so as to officiate as 

 swimming-paddles. The hinder extremity of the body is com- 

 posed of thirteen rings, which have no legs attached to them; 

 and the last segment of the tail is either a flattened plate or a 

 narrow, sword-shaped spine. Fragments of the skeleton are 

 easily recognized by the peculiar scale-like markings with 

 which the surface is adorned, and 

 which look not at all unlike the 

 scales of a fish. The most fa- 

 mous locality for these great Crus- 

 taceans is Lesmahagow, in Lan- 

 arkshire, where many different 

 species have been found. The 

 true King-crabs (Limuli} of exist- 

 ing seas also appear to have been 

 represented by at least one form 

 (Neolimulus} in the Upper Silu- 

 rian. 



Coming to the Mollusca, we 

 note the occurrence of the same 

 great groups as in the Lower 

 Silurian. Amongst the Sea- 

 mosses (Polysoa}, we have the 

 ancient Lace-corals (Fenestella 

 and Retepora}, with the nearly- 

 allied Glauconome, and species of 

 Ptilodictya (fig. 66) ; whilst many 

 forms often referred here may 

 probably have to be transferred 



tn the Cnrak incf as snmp sn Fis - 65> ~ Pterygotus Anglicv*, 



orais, JUS viewed from the under side, reduced 



called Corals will ultimately be in size and restored, c c, The feelers 



(antennae), terminating in nipping- 

 removed to the present group. claws; oo, Eyes ;mm, Three pairs of 

 The TCrarViirmnHs rrmtirmpH jointed limbs, with pointed extremi- 

 ties ; n n, Swimming-saddles, the 



to flourish during the Upper bases of which are spiny and act as 

 ., .... jaws. Upper Silurian, Lanarkshire, 



bilunan period in immense num- (After Henry Woodward.) 



bers and under a greatly in- 

 creased variety of forms. The three prominent Lower 

 Silurian genera Orthis, Strophomena, and Leptana are still 

 well represented, though they have lost their former pre- 

 eminence. Amongst the numerous types which have now 



