THE CAMBRIAN PERIOD. 



anterior of these divisions is a shield or buckler which covers 

 the head; the second or middle portion is composed of mov- 

 able rings covering the trunk ("thorax"); and the third is a 

 shield which covers the tail or " abdomen. " The head-shield 

 (fig. 31, e) is generally more or less semicircular in shape; and 

 its central portion, covering the stomach of the animal, is usu- 

 ally strongly elevated, and generally marked by lateral furrows. 

 A little on each side of the head are placed the eyes, which 

 are generally crescentic in shape, and resemble the eyes of 

 insects and many existing Crustaceans in being " compound, " 

 or made up of numerous simple eyes aggregated together. 

 So excellent is the state of preservation of many specimens of 

 Trilobites, that the numerou individual lenses of the eyes 

 have been uninjured, and as many as four hundred have been 

 counted in each eye of some forms. The eyes may be sup- 

 ported upon prominences, but they are never carried on mov- 



Fig. 31. Cambrian Trilobites: a, Paradoxidea Bohemicua, reduced In size; 6, Elllp- 

 socephalus Hoffl; c, Sao fiirsuta; d, Conocorypfie Sultzeri (all the above, together with 

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

 of Dikelloccphalua Celticus, from the Llngula Flags of Wales;/, Head-shield of Cono- 

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

 Affnostus rex, Bohemia; h, Tail-shield of Dikellocephalus M Innesotensis , from the 

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

 and Dale Owen.) 



