TRILOBITA. 



5*9 



few forms, such as Agnostus, Ampyx, some of the Trinudei, and 

 certain forms of Conocephahis. Though facets are usually easily 

 detected in such as have eyes of any size, there are some (Pronteus, 

 Arethusina, Proetus, <Scc.) in which the eyes are smooth. In any 

 case, the number of lenses varies greatly, there being as few as four- 

 teen facets, or as many as fifteen thousand in each eye in different 

 types (Barrande). 



Behind the cephalic shield comes the thorax, composed 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 Planus) 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 (Harpes ungula), 

 or twenty-nine. Ordinarily the thorax (fig. 373) is strongly trilobed, 

 and each body-ring exhibits the same trilobation, being composed of 



Fig. 371.- — Cheirurns j>leur- 

 cxantlujnus. 



Fig. 372. — Calymene 

 Bliimcnbachii. 



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

 flatter side-lobes, termed the " pleurae." The pleurae 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, Ogygia, Phacops, Calymene, &c.) In another 

 type, on the other hand (as in Cheirurus, Bronteus, Addaspis, &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 



