36 



nately more raised at the sides and more compressed on the periphery, 

 The inner faces of the whorl are more oblique and not so evenlj' rounded, 

 and the umbilicus is rather less than half the entire diameter. These 

 differences, however, may be partl3' due to the distoi'tion which the small 

 sjDecimen has undergone ; compression having been effected in a direction 

 at a right angle to the sides. The umbilical cavity is concavely and 

 rather deejDly excavated in the centre, but it gets shallower rapidly as 

 the shell increases in size. In this specimen there are not less than six 

 volutions, and perhaps as many as seven. Thecostation, in this particular 

 instance, is remarkably coarse for the size of the shell, and the inter- 

 mediate secondary ribs are almost, if not altogether, absent. On the outer 

 whorl there are twenty- four ribs, each of which-proceeds from the sutures, 

 and bifurcates about the middle of the sides. After bifurcating on one side 

 of the shell, the ribs pass over the periphery, and re-unite (in this instance 

 at least) at directly ojiposite and corresponding points, on the other. 

 The linear elevations which arise at the points where the ribs bifurcate, are 

 unusually prominent and give the shell a much more decidedly coronated 

 aspect than it bears when it has attained to nearly its full size. Two out 

 of these six small specimens shew a coarse style of ribbing, the ribs being 

 exceptionally prominent and wide apart. Sucii individuals can scarcely be 

 distinguished from the fossil figured by D'Orl^gn}^ in the ''Paleontologie 

 Frangaise " * as the Ammonites Braikenridgli of Sowerby.f In that species 

 the raised protuberances which arise where the ribs bifurcate, alternate 

 with each other across the sijjhonal edge, instead of being placed at j)oints 

 immediately opposite, but an alternate grouping may also be traced ab- 

 scurely in some of the Queen Charlotte Island specimens. 



In the four remaining, the ribs, as they arise from the sutures, are as 

 wide apart as ii. the others, but they either trifurcate before passing over 

 the periphery, or else a single secondary rib alternates with each of the 

 bifurcating primaries. Along the siphonal edge, therefore, the ribs in 

 this variety are much more numerous and closer together than they are 

 in the other. 



In all the specimens, whether large or small, the sculpture is remark- 

 ably similar. The ribs are invariabl}^ acute, they are separated by con- 

 cave grooves which are much wider than the costaj themselves, and the 



* "Terrains Jurassiques." Vol. II. Atlas, Plate CXXXV, figs. 3-5. 



t The original description and figure A. Braikenridgii in the " Mineral Conchology," are so vague 

 and unsatisfactory that it is by no means improbable that this name may have been bestowed by European 

 writers on two very different spetjies. Further, the shell represented by Pictet, in his "Traite de Pal6on- 

 tologie " (Atlas, Plate LV., fig. 1) as AiMuonites Humphreysianus, seems to be identical with the^. 

 Braikenridgii of D'Orbigny. 



