CANADIAN FOSSILS. 29 



The ambulacral orifices in this species are indicated by a strong, 

 narrow, rounded channel in the centre of the upper edges of the 

 primary radial plates. 



Explanation of Figures. Plate II. 



Figure 4a. View of the anterior side of a specimen, shewing the azygos plates. 

 " lb-Id. Different views of other specimens. 

 " le. A polished transverse section just below the base of the arms, shewing 



the channels in the primary radial plates for the ambulacral 



orifices. 



Locality and formation. — Trenton limestone ; City of Ottawa. 



Hyboceinus conicus, Billings. 

 Plate II. Figures 2a-2b. 



(H. CONICUS, Report Geological Survey of Canada, 1856, page 2*74.) 



Description. — In this species the cup is conical, with slightly 

 ventricose sides ; the base narrow, and the arms long and undivided ; 

 plates, smooth ; height of cup, thirteen lines from the base on the 

 azygos side to the upper margin ; height of the opposite side, nine 

 lines; length of the arms, three inches; the basal plates occupy 

 more than one half the height on the azygos side and about one half 

 on the others ; the arms are one line and a half in width, broadly 

 rounded on the back, and composed of a single series of joints, 

 each one line in length ; on their insides the ambulacral grooves are 

 margined by rows of small plates resembling those upon the arms 

 of some of the Cystideae (Pleurocystites) ; about five of those plates to 

 one joint of the arm. The column is round and smooth, consisting 

 of very thin joints, ten to one line. The mode of attachment to the 

 bottom was by a broad button-shaped base. Length of column in 

 the largest specimen seen, one and three-quarter inches. 



Explanation of Figures. Plate II. 



Figure 2a. Right side of a specimen of the average .size. 

 " 26. Left side of another specimen. 



Locality and formation. — Trenton limestone. City of Ottawa. 



