OF CENTRAL CANADA PART IV. 



257 



FIQ. 172 



Trinucleus concentricus 

 Trenton and Hudson 



forated margin ; Cheirurus or Ceraurus, with eleven body rings, and 

 facial-suture terminating at the sides of the head-shield ; Encrinurus, 

 also with eleven body-segments and other characters as in Cheirurus, 

 but with many-ringed axis to its pygidium ; and Dalmannites and 

 Phacops, forms with eleven body-segments and facial suture also as 

 in Cheirurus, with other characters described below. 



Paradoxides is strictly a Cambrian type. Examples occur in New 

 Brunswick and Newfoundland, but none appear to have been dis- 

 covered elsewhere in Canada. Trinucleus is essentially a Silurian 

 type, characterised by its six body-segments, its 

 globose, strongly-pronounced glabella, and the 

 perforated border of its head-shield. Adult 

 forms are without eyes. Figure 172 shews our 

 common species T. concentricus from the Tren- 

 ton and Hudson River (Lower Silurian) forma- 

 tions. Ceraurus is also essentially a Silurian 

 type, although the genus ranged from the Upper 

 Cambrian into the Devonian period. It is 

 characterised chiefly by its eleven body-segments, River Formations, 

 its pleurae with raised bands in place of furrows, and the lateral termina- 

 tion of its facial sutures. By the latter character as well as by the 

 number of its body-segments, it approaches Phacops and Dalmanites ; 

 but from these it is distinguished among 

 other characters, by its finely reticulated 

 eyes, its glabella of almost uniform width, 

 and the bands upon its pleurae. Figure 173 

 represents a common species, C. pleurexanthe- 

 nus from our Trenton limestone. Impress- 

 ions of the glabella and two-horned pygi- 

 dium, are especially abundant in some of 

 these limestone beds, as at Belleville, Peter- 

 borough and elsewhere. Phacops is char- 

 acteristic both of Silurian and Devonian 

 strata. It is distinguished by its eleven 

 body-segments; its laterally-terminating 

 Ceraurus ( = Cheirurus) pien- facial suture; its large anteriorly expanded 



facetted eyes ; and its roundedipleurse. A common Devonian species, 

 17 



