198 



Arthropoda. .Class 1. Crustacea. 



remotely related to the King-crabs are the genera Eurypterun (Silurian) 

 and otherSj witli a relatively smaller cephalothorax^ with five pairs of 



Pig. 159. A Limulus, B Belinurus, C Eurypterus, D a Trilobite {Dalmanites 



locomotor appendages, bearing a general resemblance in size and 

 shape to those of Limulus, and with a large jointed abdomen and a 

 short caudal-spine. 



Order 4. Trilobita. 



The flat, oval body of the Trilobites is divided into three regions 

 {see Pig. 159 D) ; cephalothorax, thorax, and abdomen, of which the 

 thorax is usually the largest. 



The cephalothorax is unsegmented, its anterior and " lateral 

 edges are curved, but its posterior one straight ; the lateral angles are 

 not infrequently prolonged into backwardly directed spines. Placed 

 close together on the dorsal side, there is usually a pair of large, 

 sessile, compound eyes. The thorax consists of a number (2 — 26) 

 of movable, short, wide segments. The abdomen is formed of 



a number of fused segments, whose 

 limits are sometimes clear, but 

 usually indistinguishable. Two 

 longitudinal furrows traverse almost 

 the entire length of the dorsal side, 

 from one end of the body to the 

 other, marking out the surface into 

 a median (t e r g u m) and two 

 lateral areae (p 1 e u r se) . The ventral 

 surface, with the appendages, was probably very soft and thin- 

 skinned, whilst" the dorsal was hard. In only a few cases have 

 indubitable traces of the ventral surface and the limbs been found. 



Fig. 160. — Stages in the deTelopment of 

 a Trilobite {Sao hirsuta). — After Barrande. 



