CRUSTACEA. TRILOBITA 297 



enable the animal to roll itself up like a woodlouse. 

 Each segment is divided into a median and two lateral 

 parts by means of two furrows. The median or axial 

 part is more convex than the lateral, and forms the axis 

 (i), the lateral parts being known as the pleiwce (k). The 

 anterior part (fig. 127,c) of 

 the axis of each segment 

 is not visible when the 

 animal is unrolled, since it _. .,_ _ , „ „ , 



.big. 127. Dorsal surface of a thoracic 

 IS bent down and IS Over- segment of Asaphus expansus. a, 



lapped by the preceding ring of ax^; &, groove; c, articular 



rr J i o portion ; a, furrow between axis and 



Segment, for which it pleura ; d—g, pleura ; e, fulcrum ; 



( . . ■, P f, facet ; h, groove on pleura, 

 forms an articular surface. 



The pleurae in some genera possess a longitudinal ridge, 

 in others a groove (h), or both ridge and groove may 

 occur; a few forms have plane pleurae. Each pleura, at 

 some distance from the axis, is curved downwards and 

 usually also backwards ; the point where this curvature 

 occurs is known as the fulcrum (e) ; sometimes the outer 

 part of each pleura overlaps the anterior part of the suc- 

 ceeding one, and then the front part of the pleura beyond 

 the fulcrum may be smooth and flattened so as to form an 

 articulating surface or facet (f). The terminations of the 

 pleurae are in some cases rounded (fig. 127), in others 

 pointed or produced into spines (fig. 130). 



The pygidium or abdomen (fig. 123 C) is commonly 

 triangular or semicircular in shape, and is formed of a 

 variable number of segments, which differ from -those of 

 the thorax in being fused together and immovable ; on 

 the dorsal surface the segmentation is shown by grooves 

 only. The pygidium, like the thorax, is divided into a 

 median part or axis, and lateral portions. The axis may 

 reach quite to the posterior extremity or only part of the 



