211 



OLCOSTEPHANUS LOGANIANUS, Whiteaves. 

 Plate 23, figs. 1 and la. 



Ammonites Loganianus, Whiteaves. Type. This volume, p. 27, pi. 8, fig. 2. 



StepJianoceras Humplireysianum, Hyatt. Geological Survey of Canada, Re- 

 port of Progress, 1876-77, p. 156: 

 but not Ammonites Humphreysia- 

 nus, Sowerby. 



South side of Alliford Bay, in Skidegate Inlet : four well preserved 

 but imperfect specimens. 



The type of Ammonites Loganianus is a badly distorted cast, which 

 gave a very incorrect idea of the number of the volutions of the shell, 

 of the true amount of their involution, and consequently of the pro- 

 portionate width of the iimbilicus. A nearly perfect specimen of this 

 species has since been collected by Dr. G. M. Dawson, at Sigutlat Lake, 

 B.C., which is figured on plate 23, and this, together with the specimens 

 from Alliford Bay, enable the description of its characters given on 

 pages 27-29 of the present volume to be amended as follows : 



When perfect and undistorted, the shell appears to have been com- 

 posed of about five rounded volutions, which are so lightly enrolled 

 that more than one-half of the sides of the inner ones are exposed. 

 In some specimens the outer whorl is somewhat compressed on the 

 periphery, and in others the sides are slightly compressed. The um- 

 bilicus is broad and open, but its margin is indistinctly defined. A row 

 of tubercles on the last volution, from which the primary ribs tri- 

 furcate, appears to represent its outer boundary, and assuming this to 

 be the case, then the maximum width of the umbilicus is equal to 

 fully two-thirds of the entire diameter. The aperture is usually, 

 though not always, broader than high, transversely reniform or sub- 

 crescentic in outline, and shallowly as well as concavely emarginated 

 at its base by the encroachment of the preceding whorl. The surface 

 is regularly ribbed, and the costation consists of primary trifurcating 

 ribs, with one or two secondary ribs intercalated between each pair of 

 primaries. On the outer whorl the primary ribs commence at the 

 suture and extend nearly half-way across the sides to the outer 

 boundary of the umbilicus, as simple, broad, and distant costa?; then 

 at the umbilical margin each primary rib trifurcates from a trans- 

 versely elongated and rather prominent tubercle before passing over 

 the periphery. On the same volution the secondary ribs are confined 

 to the outer half of the sides. From this disposition of the ribs it 

 follows that there are usually four or five times as many on the 

 periphery as there are between the umbilical margin and the suture. 



