CRUSTACEA. TRILOBITA 293 



segments of the head and of the pygidium are fused 

 together, but those of the thorax remain free. Traces of 

 the alimentary canal are sometimes found in the middle 

 or axial part of the Trilobite. 



The dorsal surface of the body is protected by a strong, 

 calcareous exoskeleton. The part which covers the head 

 is known as the head-shield or cephalic shield, and is 

 usually semicircular or triangular in shape ; in it may 

 be distinguished a median and two lateral portions ; the 

 former is the more convex and is termed the glabella (a), 

 the latter are the cheeks. The glabella is marked off from 

 the cheeks by means of a furrow on each side, known as 

 the axial furrow (a). The form and relative size of the 

 glabella vary in different genera; in some it extends quite 

 to the anterior margin of the head-shield, in others only a 

 part of the way (fig. 131); sometimes it is wider behind 

 than in front ; in other cases it is wider anteriorly, or it 

 may be of uniform width throughout ; its convexity also 

 varies considerably — it may be nearly flat, but is some- 

 times pear-shaped or spheroidal. The segmentation of 

 the head is indicated by transverse furrows on the glabella 

 (b), — often three on each side ; in some cases the opposite 

 furrows from the two sides meet at the middle of the 

 glabella. On the posterior part of the glabella there is 

 another furrow, which extends quite across it and is con- 

 tinued on to the cheeks ; this is known as the neck-furrow 

 (b'), and the segment of the glabella behind it is the neck- 

 ring. These furrows indicate the existence of five seg- 

 ments in the head. In primitive trilobites all the furrows 

 are distinct, but in others some, especially the anterior 

 furrows, become indistinct or obsolete. 



The cheeks are more or less triangular in shape, and 

 usually less convex than the glabella ; they are frequently 



