Section I\', 1921 [47J Traxs. R.S.C. 



On Triarthrus canadensis , Triarthriis glaber, and Triarlhnis 



spinosus 



By W. A. Parks, Ph.D., F.R.S.C. 



(Read May Meeting, 1921) 



Triarlhriis canadensis was described by J. F. Smith, Jr., in a 

 paper read before the Canadian Institute on January 26, 1861, and 

 published in the Canadian Journal, Volume V^I, page 275. Billings 

 mentions the species in his account of Triarthriis fischeri (Palaeozoic 

 Fossils, Vol. I, page 291) and includes it in a list of Utica fossils on 

 page 953 of the Report of Progress of the Geological Survey of Canada 

 for 1863. Barrande refers to T. canadensis in Système Silurien de la 

 Bohême, Vol. I, Supplement, page 429. In the Canadian Record of 

 Science, Volume V, page 175, Ami says: " Triarthriis canadensis, with 

 its peculiar genal angle produced into a prominent spine on each side 

 of the head, is most abundant in the Utica shales of the islands in 

 the northern portion of Lake Huron, such as the islands north of 

 Maple Cape, etc." Bassler lists the species as a Collingwood fossil 

 in "American Ordovician and Silurian Fossils." 



The holotype, obtained by Smith at Whitby, Ontario, consists 

 of the head only, with both free cheeks preserved but with the axial 

 portion imperfect. The author refers, also, to a single free cheek 

 of larger size but of indifferent preservation. The essential char- 

 acteristic of the species is the possession of stout genal spines directed 

 backwards and outwards. "The only other species having long 

 spines is Triçrthrus spinosus (Billings). By reference to Mr. Billings' 

 description the difference in T. ca^iadensis will become at once ap- 

 parent. The horns of the former are slender and cylindrical and 

 point with a slight curve, almost directly downwards to the eighth 

 pair of pleurae. In T. canadensis they are flattish and rather thick, 

 with a groove running down the centre, and they extend at an angle 

 of about 40°, evidently not farther than the fourth pair of pleurae." 



The above description is accompanied by a rather poor wood- 

 cut of the head. No description is given of the rest of the animal 

 nor do any of the references given above contain additional informa- 

 tion as to the morphology of the species. As far as I can ascertain a 

 whole specimen has not hitherto been described. 



On the Rouge river, about fifteen miles east of Toronto, are three 

 exposures of shale filled with detached cranidia and pygidia of two 



