304 ZOOLOGY. 
the representatives of the suborder Xiphosura. The second 
suborder Hurypterida is represented by extinct genera Ptery- 
gotus, Hurypterus 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. The 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 
Hurypterida 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 Arachnida as 
well as to the genuine Crustacea, resembling the former in 
the want of antenne, and their mode of development. 
Order 2. Trilobita.—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. The 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 (pygidium), 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 (Paradozides, 
Agnostus, etc.) appeared in the lowest Cambrian strata, cul- 
minated in the upper Silurian, and died out at the close of 
the Coal Period. 
