THE CAMBRIAN PERIOD. 85 



in some of the Cambrian Trilobites, such as the little Agnosti 

 (lig. 31 g), the animal was blind. The lateral portions of the 



Fig. 31. Cambrian Trilobites: a, Paradorides Bohemicns, reduced in size; b, Ellifi- 

 socephalus Hoffi; c, Sao hirsTita; d, Conocoryphe Snttzeri (all the above, together with 

 fig. ?, are from the Upper Cambrian or "Primordial Zone" of Bohemia); e, Head-shield 

 of Dikellocephaltis Celticits, from the Lingula Flags of Wales ; /, Head-shield of Cono- 

 coryphe Mattheiui, from the Upper Cambrian (Acadian Group) of New Brunswick; g, 

 Agnostus rex, Bohemia ; h. Tail-shield of Dikellocephahts Minnesotensis, from the Upper 

 Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande, Dawson, Salter, and 

 Dale Owen.) 



head-shield are usually separated from the central portion by 

 a peculiar line of division (the so-called " facial suture ") on 

 each side ; but this is also wanting in some of the Cambrian 

 species. The backward angles of the head-shield, also, are 

 often prolonged into spines, which sometimes reach a great 

 length. Following the head-shield behind, we have a portion 

 of the body which is composed of movable segments or "body- 

 rings," and which is technically called the " thorax." Ordi- 

 narily, this region is strongly trilobed, and each ring consists of 

 a central convex portion, and of two flatter side-lobes. The 

 number of body-rings in the thorax is very variable (from two 

 to twenty-six), but is fixed for the adult forms of each group of 

 the Trilobites. The young forms have much fewer rings than 

 the full-grown ones ; and it is curious to find that the Cam- 



