TRILOBITA. 



355 



of a variable number of segments which are not soldered 

 together, but are capable of more or less movement upon 

 each other. The amount of movement thus allowed varies, 

 but in several genera (e.g., Calymene and Illcenus) it was 

 sufficiently great to allow of the animal completely rolling 

 itself up after the manner of a hedgehog. The number of 

 body-rings or segments in the thorax varies from no more 

 than two (Agnostus) to as many as twenty-six (ffarpes ungulct). 

 Ordinarily the thorax (fig. 211) is strongly trilobed, and each 

 body-ring exhibits the same trilobation, being composed of a 



Fig. 208. Bronteus luna 



Fig. 209. CheiruriM pleur- 

 exanthemus. 



Fig. 210. Calymene 

 Blumenbachii. 



central, more or less convex portion, called the " axis," and 

 of two flatter side-lobes, termed the "pleurae." The pleura? 

 are in one piece with the axis, but are separated from it by 

 a more or less pronounced groove, the ^axal furrow." In 

 one type of pleurae, each of these structures carries a deep 

 longitudinal groove or sulcus upon its upper surface (as in 

 Asaphus, Oyygia, Phacops, Calymene, &c.) In another type, 

 on the other hand (as in Cheirurus, Bronteus, Acidaspis, &c.), 

 the place of the sulcus is taken by a similarly situated oblique 

 ridge. The pleurae are always bent downwards towards their 

 ends, and also commonly bent backwards as well, the point 

 where the backward curvature begins being the " fulcrum " 

 of Salter. In the Trilobites with grooved pleurae, more 

 especially, the pleurae imbricate and overlap ; and the " ful- 



