48 THE ROYAL SOCIETY OF CANADA 



distinct species of Triarthrus. One whole specimen and many free 

 cheeks of a variety of T. spinosus were obtained; also, one good 

 specimen and a number of free cheeks of T. canadensis. It is a 

 reasonable inference that the numerous cranidia and pygidia belong 

 to these two species. Slabs of shale from Craigleith, Ontario, a 

 classic locality for Utica and Collingwood fossils also contain numerous 

 free cheeks of T. canadensis. 



The almost perfect specimen of T. canadensis (Figure 2) is 

 18 mm. long, but the largest free cheek found indicates a length of 

 43 mm. The measurements are as follows: 



mm. 



Total length . 18.5 



Length of head (median) 5.0 



Width of head 15.0 



Length of thorax 11.0 



Maximum width of thorax 10.5 



Length of pygidium 2.5 



Width of pygidium 5.9 



The axial portion is rather more than one-third the total width 

 and the axial furrows are well defined. The body is distinctly arcuate 

 in the thoracic region with the three lobes sharply differentiated. 

 While the cast of the inside of the dorsal crust, with some very thin 

 shell adhering, seems to be quite smooth the mould of the upper 

 surface indicates a minute tuberculation which is most pronounced 

 in the pleural regions. 



The head is strikingly like that of T. glaber in its general outline 

 and in the proportions of its various parts; in fact, it differs only 

 in the prolongation of the genal angles into spines. The glabella is 

 well defined, narrowing slightly forwards, and rounding abruptly 

 into the rather straight anterior margin. The occipital furrow is 

 conspicuous, concave anteriorly, and with a rather sharp anterior 

 direction near the axial furrows. The two anterior glabellar furrows 

 are convex anteriorly with the two halves well separated at the centre. 

 The facial suture begins well within the genal angle on the 

 posterior margin, turns slightly outwards, then concavely inward at 

 the marginal furrow, slightly outwards below the eye and rather 

 sharply inwards immediately behind the eye, convexly outwards along 

 the margin of the palpebral lobe, and apparently terminates on the 

 anterior margin but the course of the suture forward from the eye is 

 not clearly revealed. 



While this course of the facial suture is distinctly shown on the 

 cast the mould indicates quite as clearly a more simple sigmoidal 



