324 ZOOLOGY. 



the representatives of the suborder Xijjhosura. The second 

 suborder Eurypterida is represented by extinct genera Ptery- 

 gotus, Eurypterus and allies which appeared in the upper 

 Silurian Period and became extinct in the Coal Period. In 

 these forms the cephalothorax is small, flattened and nearly 

 square, while the abdomen is long, with twelve or thirteen 

 segments, the last one forming a spoon-shaped or acute 

 spine. Tlie appendages of the cephalothorax were adapted 

 for walking, one pair sometimes large and chelate ; the 

 hinder pair paddle-like. The gills were arranged like the 

 teeth in a rake, the flat faces being fore and aft. While the 

 king-crab burrows in the mud and lives on sea-worms, the 

 Eurypterida probably swam near the surface, and were more 

 predatory than the king-crabs. The Merostomata are a gen- 

 eralized type, with some resemblances to the Araclinida as 

 well as to the genuine Crustacea, representing the larval or 

 Zoea form of the Deoajjods. 



Order 2. TriloMta. — The members of this group are all 

 extinct. The body has a thick dense integument like that 

 of Limulus, and is often variously ornamented with tuber- 

 cles and spines. The body is divided into three longi- 

 tudinal lobes, the central situated over the region of the 

 heart as in Limulus. Tlie body is more specialized than in 

 the Merostomata, being divided into a true head consisting 

 of six segments bearing jointed appendages, somewhat like 

 those of the Merostomata, with from two to twenty-six dis- 

 tinct thoracic segments (probably bearing short jointed limbs 

 not extending beyond the edge of the body, which support- 

 ed swimming and respiratory lobes). The abdomen consisted 

 of several (greatest number twenty-eight) coalesced segments, 

 forming a solid portion {p>ygidium), sometimes ending in a 

 spine, and probably bearing membranous swimming feet. 

 The larval trilobite was like that of a king-crab, and after a 

 number of moults acquired its thoracic segments, there being 

 a well-marked metamorphosis. The Trilobites {Paradoxides, 

 Agnostus, etc.) appeared in the lowest Silurian strata, cul- 

 minated in the upper Silurian, and died out at the close of 

 the Coal Period. 



