Fossils of the Calciferous Sandrock. 353 



Description. — Lenticular, with a sharp elevated margin ; spire 

 much depressed; apical angle varying from 130° to 145° ; whorls 

 four or five, rather slender with a thin elevated outer edge which 

 is more or less distinctly visible all the way to the apex ; they are 

 also slightly concave on their upper surface, the concavity being 

 deepest near the lower edge ; in some specimens there is a gentle 

 convexity in the upper half of each volution and in others the 

 surface is nearly flat. On the under side of the shell the 

 whorls are convex and appear to have a prominent obtuse angle 

 at the edge of the umbilicus ; the latter is large, conical, penetrat- 

 ing to the apex and at least half as wide as the whole shell. The 

 surface is covered with fine striae which turning backward at an 

 acute angle indicate a deep notch in the outer angle of the aper- 

 ture. The last whorl in some of the specimens on approaching the 

 aperture drops a little below the margin of the second, shewing a 

 tendency to become disengaged. The aperture judging from the 

 form of the volutions must be sub-rhomboidal. The largest speci- 

 men seen is one inch and a half wide, and about half an inch in 

 height. 



This species is closely allied to R. aperta (Salter), but differs 

 therefrom in the following particulars. 1st. The height R. aperta 

 is about half the width, in this species about one third. 2nd. In 

 R. aperta the edges of the whorls on both sides of the suture sink 

 below the general surface, and there is consequently a rather deep 

 spiral channel running to the apex, but in this species the outer 

 edge of the whorl rises above the general surface and forms an 

 elevated spiral line following the upper side of the suture from the 

 aperture to the apex. 3rd. In R. aperta the inner half of the 

 width of the volution is always strongly convex and the outer 

 half as strongly concave but in R, calcifera the whole surface of 

 the whorl is concave or only very slightly convex towards the inner 

 side. 



As all the specimens that I have seen are embedded in the 

 rock, I have only been able to ascertain the characters of the 

 underside of the shell from fragments. The base seems to be 

 more like R. aperta than the spire. The two species are closely 

 allied and should, perhaps, be considered as varieties of each other. 



Locality and formation. — Near Beauharnois, Calciferous Sand- 

 rock. 



Collector. — J. Richardson. 



Canadian Nat, 3 Vol. IV. No. 5. 



