124 



HISTORICAL PALEONTOLOGY. 



highest and most beautiful of the groups of Trilobites, attaining 

 here its maximum of development. In the annexed illustra- 

 tion (fig. 64) some of the characteristic Upper Silurian Trilo- 



Fig. 64. Upper Silurian Trilobites. a, Cheirurus bimucronatus, Wenlock and Cara- 

 doc; b, Phacops longicaudalus, Wenlock, Britain, and America; c, Phacops Downingice, 

 Wenlock and Ludlow ; d, Harpes ungula, Upper Silurian, Bohemia. (After Salter 

 and Barrande.) 



bites are represented all, however, belonging to genera which 

 have their commencement in the Lower Silurian period. In 

 addition to the above, the Ludlow rocks of Britain and the 

 Lower Helderberg beds of North America have yielded the 

 remains of certain singular Crustaceans belonging to the 

 extinct order of the Eurypterida. Some of these wonderful 

 forms are not remarkable for their size ; but others, such as 

 Pterygotus Anglicus (fig. 65), attain a length of six feet or more, 

 and may fairly be considered as the giants of their class. The 

 Eurypterids are most nearly allied to the existing King-crabs 

 (Limuti), and have the anterior end of the body covered with 

 a great head-shield, carrying two pairs of eyes, the one simple 

 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 



