124 HISTORICAL PALAZZONTOLOGY. 
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, Chetrurus bineucronatus, Wenlock and Cara- 
doc; 6, Phacops longicaudatus, Wenlock, Britain, and America; c, Phacofs Downingia, 
Wenlock and Ludlow; ad, Harfes 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 Lurypterida. Some of these wonderful 
forms are not remarkable for their size; but others, such as 
Prerygotus 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 
(Zzmulz), 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 
