82 
GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. 
Gallery 
VIII. 
Table-ease 
25. 
Class TRILOBITA. 
Tlie structure of the Trilobites (tliree-lobed forms) is 
illustrated by models. Dalmanites caudatus (Fig. 38) shows 
Head-shield 
Tliorax - - 
PyKldium- - 
rosinim 
•jlabella 
free cheek 
axal furrow 
eye lobe 
eye 
facial mtxire 
fixed cheek 
neck furroio 
yenal sjnne 
axis 
pie won 
pleural groove 
margin 
axal furrow 
mucro 
Fig. 38. — Diagram of a Trilobite, Dalmanites {Phacops'] caudatus, Won- 
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 
hoi’iiy 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, gi-adually 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 
pygidiuin, of about fourteen segments. The segments of the 
