82 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery CLA.SS TRILOBITA. 



VIII. 



'^^^^-^^^^ The structure of the Trilobites (three-lobed forms) is 

 illustrated by models, Dalmanites caudatus (Fig. 38) shows 



Head-shield 



Thorax - - - / 



Pygidium ■ 



genal spine 



axis 

 ~ pleuTon 

 - pleural groove 



Fig. 38. — Diagram of a Trilobite, Dalmanites [Pliacojps] caudatus, Wen- 

 lockian. Where the black lines are drawn across is a definite suture 

 or joint. The five segments of the head-shield are numbered. 



the ordinary view, namely, the back or upper side of the 

 horny coat. Two grooves running lengthwise divide it into 

 three lobes — a middle or " axial," and two side or "pleural " 

 lobes. The principal organs of the body lie within the axial 

 lobe, while the pleural lobes are expansions of this for the 

 protection of the limbs. At first glance the coat appears 

 made of several (about thirty) segments, gradually decreasing 

 in size and importance from head to tail Inspection shows, 

 however, that these segments are more closely united into 

 three regions : the head-shield, composed of five segments ; 

 the thorax, of eleven segments ; and the tail- shield or 

 pygidium, of about fourteen segments. The segments of the 



