WH.TEAVES.] DEVONIAN FOSSILS, MACKENZIE RIVER BASIN. 215 



three specimena, attached to the ventral valve of Strophodonta demissa. 

 Athabasca Eiver, three niiles below the Calumet, E. G, McOonnell, 

 1890: one upper or dorsal valve. 



Ohonetes Logani, var. Aurora, Hall. 

 Plate 29, figs. 2, 2a. 



Chonetes Logani, var. Aurora, Hall. 1867. Pal. St. N. Y., vol. IV, pt. 1, p. 137, 

 pi. xxii, figs. 16-28. 



Athabasca Elver, opposite La Saline, Professor Macbun, 1875, and E. 

 G-. McConnell, 1890,— also in the tirst ton miles below the Clearwater, 

 about fifteen or twenty miles farther up, Dr. E. Bell, 1882 : a small piece 

 of limestone, from each of these localities, strewn with detached valves 

 ot this species, the dorsal valves being better presej'ved than the ven- 

 tral. Mackenzie Eivei', at the "Eamparts," six large and unusually 

 well preserved specimens of the ventral valve, one of which is figured, 

 and three of the dorsal : also, on the same river, six miles below 

 "Eock by the river side," a few partially exfoliated v^alves on a small 

 piece of rock. 



As the specimens from these localities appear to present some slight 

 differences from the types of C. Logani, var. Aurora from New York 

 and Ohio, the former may be characterized as follows : — 



Shell )-ather small, concavo-convex or plano-convex, about one 

 fourth broader than long, transverselj^ semielliptical in marginal out- 

 line, cardinal border about equal to or a little less than the greatest 

 breadth of the valves. Ventral valve moderately convex, its umbo 

 broad and somewhat tumid, its beak small, depressed and incurved. 

 On each side of the beak the cardinal edge is armed with three widely 

 divaricating, long and slender sjjines. When viewed under a lens, the 

 surface of the ventral valve is seen to be marked; with numerous 

 minute, but distinct and somewhat flexuous, round] and radiating, 

 thread-Iiie raised lines which increase, by bifurcation and intercala- 

 tion, from about fifty on the umbo to sixty near the front margin. 

 These are crossed by slightly smaller and more close set, but in other 

 respects similar, concentric raised lines (and occasionally also, by a 

 few distant and comparatively coarse concentric wrinkles) in such a 

 way as to produce a minutely nodulous network, though the radiating 

 raised lines are more widely separated than the concentric ones, 

 and the latter are sometimes interrupted, or in some cases dichotomous. 

 Dorsal valve shallowly concave or nearly flat, its outer surface marked 



