THE CAMBRIAN PERIOD. 85 



in some of the Cambrian T^ilobites, such as the little Agnosti 

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



Fig. 31. Cambrian Trilobites : a, Parado rides Bohemicus, reduced in size; b, Ellifi- 

 socephalus Hoffi ; c, Sao hirsuta ; d, Conocorypke Sitltzeri (all the above, together with 

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

 of Dikellocephalus Celticiis, from the Lingula Flags of Wales; f, Head-shield of Coiio- 

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

 Agnostns rex, Bohemia ; //, Tail-shield of 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 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- 



