THE CAMBRIAN PERIOD. 



85 



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

 (fig. 31 g)^ the animal was blind. The lateral portions of the 



Fig. 31. — Cambrian Trilobites: a, Paradoxides Bokemicus, reduced in size; b, Ellip- 

 socephaltis Hoffi ; c, Sao hirsjita ; d, Conocorypke Sultzeri {zSS. the above, together with 

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

 Qii Dikellocephalus Celticus, from the Lingula Flags of Wales; f, Head-shield o{ Cono- 

 corypfie Matthewi, from the Upper Cambrian (Acadian Group) of New Brunswick; g, 

 Agnosttis rex, Bohemia ; h, Tail-shield o^ Dikellocephalus 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 pecuHar 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- 



