OF CENTRAL CANADA PART IV. 



259 



FIG. 176. 



lites is a Cambrian and Lower Silurian type, but as regards the 

 area treated of in the present volume it 

 occurs only in fragmentary examples in 

 the Levis formation of Quebec. It pos- 

 sesses fourteen or fifteen body-segments 

 and a glabella which narrows anteriorly 

 and has short lateral furrows as in Ca y 

 mene, a genus with which in other respects 

 it has close affinities. The genus Triar- 

 thrus is peculiar to this continent, and its 

 species are apparently confined to the 

 Lower Silurian, Utica formation. The T 



HarpesOttawaensis : after Billings. 



body-segments vary from fourteen to six- Trenton Formation 



teen in number, and in our most common species T. Beckii, each seg- 

 ment bears in the centre a short spine, as shown in figure 177. In 

 another species, T. spinosus (Billings), a very long spine is attached 

 to the eighth or ninth thoracic segment, and another to the neck seg- 

 ment.* The glabella in this genus is of nearly uniform 

 width, not much raised, and is marked on each side 

 by several short furrows. Impressions of the glabella 

 of T. Beckii occur in the shale beds west of Colling- 

 vood and at Whitby and Ottawa, in great abundance. 

 A third Canadian species, T. glaber, is destitute of 

 spines. The genus Calymene is exclusively Silurian, 

 its species are about equally numerous in the Lower 



and Upper Silurian beds, and several range from the 



FIG. 177. 

 lower into the higher series. The genus is distin- Triarthrus Beckii: 



guished by its thirteen body-segments ; its lobed gla- E Fonnation. Ca 

 bella, narrowing upwards ; and by the posterior ends of its facial 

 suture terminating at the corners of the head-shield. Our common 

 species is C. Blumenbachii ( = C. senaria) with rounded head-angles 

 and pleurae, strongly lobed glabella, and little apparent distinction 

 between the end of the thorax and commencement of the pygidium. 



* See a revised description of this species by Henry M. Ami, of the Geological Survey, 

 in a paper on the Utica Slate Formation published in the Transactions of the Ottawa Fiel 

 Naturalists' Club : 1882. 



