126 



HISTORICAL PALAEONTOLOGY. 



vidually numerous, there is a great diminution in the number 

 of generic types. Many of the old genera, which flourished 

 so abundantly in Lower Silurian seas, have now died out; 

 and the group is represented chiefly by species of Cheirurus, 

 Encrinurus, Hordes, Proetus, Lichas, Acidaspis, Illanus, Caly- 

 mene { Homalonotus, ?.nd Phacops the last of these, one of the 

 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- 

 bites are represented all, however, belonging to genera which 

 have their commencement in the Lower Silurian period. In 



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

 Caradoc ; 6, Phacops longicaudatus, Wenlock, Britain and America ; c, Phacopt 

 Downinglce, Wenlock and Ludlow ; d, Harpea ungula, Upper Silurian, Bohemia. 

 (After Salter and Barrande.) 



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 

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

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



