[parks] TRIARTHRUS-CANADENSIS, GLABER, SPINOSUS 51 



numerous cranidia all remarkably alike and all differing distinctly 

 from the cranidia of T. caîiadensis. Further, a single rather imperfect 

 specimen of T. spinosiis was found in which the cranidium agrees 

 almost exactly with those referred to above. I am forced to the 

 conclusion that the single specimen and the detached cranidia repre- 

 sent a \'ariety of T. spinosus which is described below as T. spinosiis 

 rougensis. 



The striking feature of the variety is the absence of a spine on the 

 neck segment and the occurrence of a very strong spine on the ninth 

 thoracic ring. The genal spines are normal. The failure of the 

 occipital spine on the single whole specimen might be attributed to loss 

 were it not for the fact that no trace of a spine is found on any of the 

 numerous detached cranidia. 



The single specimen referred to is shown with considerable 

 restoration in Figure 8. The general proportions are correct but 

 the pleural margins in the anterior thoracic region have been 

 restored. The posterior part of the facial suture is somewhat doubt- 

 ful; judging from the detached free cheeks the suture does not cut 

 the posterior margin immediately within the base of the spine as 

 shown. The narrow first thoracic ring is possibly due to displace- 

 ment. 



The detached cranidia ascribed to this species and variety are 

 shown by a typical example in Figure 7, and a typical cranidium 

 of T. spinosus from Cummings Bridge, Ottawa, in Figure 9. 



The Rouge exposures show, also, two types of detached pygidia. 

 There is little doubt that Figure 10 represents the pygidium of Triarthrus 

 canadensis and Figure 11 that of Triarthrus spinosus rougensis. 



The strata exposed on the Rouge river consist of dark and light 

 silty shales, in some places arenaceous, and in others presenting the 

 greyish appearance of typical Lorraine shale. There can be little 

 doubt that, petrographically, these beds show a transition from Utica 

 to Lorraine. 



Associated with the two species of Triarthrus are vast numbers of 

 flattened orthoceratites, large and small, and the following recognizable 

 species : 



Clidophorus cf.foerstei, Ruedemann. 



Prolobella trentonensis, Conrad. 



Leptobolus insignis, Hall. 



Ctenodonta cf. pulchella, Emmons. 



Cryptolithus tessellatus, Green. 



Cf. Trocholites ammonius, Conrad. 



Diplograptus sp. 



