16 CANADIAN FOSSILS. 



convex, the umbilicus deep and rather narrow, bounded by a sharp 

 edge which cuts the lower angle of the mouth about its middle. 



The mouth itself is rounded, very deeply notched at the upper 

 angle, and the edge then curves strongly forward and forms a pro- 

 jecting outer lip. The inner margin again retreats very considerably, 

 and has a sharp, almost cutting edge. The lines of growth in the 

 umbilicus follow the curve of the edge of the inner lip, which is 

 somewhat excavated, as in species of Natica, Liltorina littorea, etc., 

 by the reflection of the mantle's edge. 



The entire form is very compact, and the smooth lower surface 

 being convex, with small, deep, regular umbilicus, the shell has a 

 good deal the aspect of a land-snail, a resemblance at once contra- 

 dicted by the angular umbilicus, the sharp retreating inner lip, and 

 the notched and projecting outer one. 



The last two species violate in some respects the character of the 

 sub-genus, having the small umbilicus of Raphistoma, with an angu- 

 lar edge. In truth they might be included with the former sub-genus, 

 and are retained here on account of the rounded, not triangular mouth. 

 Raphistoma is better typified by such species as have a flat or regularly 

 conical spire, a triangular mouth, and a small umbilicus. It is 

 always difficult to define the limits of sub-genera, for they are mostly 

 convenient, rather than absolute, groups. 



Ophileta, Vanuxem. 



The characters given by the author of the genus merely describe 

 its general appearance, and do not indicate its relations to Raphistoma 

 or the other allied sub-genera. It appears to be strictly confined 

 to the lowermost of the calcareous strata which lie upon the Potsdam 

 sandstone, and holds the same place in Scotland as it does on the 

 American continent. 



0. compacta, Salter. 



Plate III. 



O. magna, sesquiuncialis, anfractibus 5-G, utrdque perangulatis, supra 

 profunde excavatis, infra planissimis ; ore trapezoidali fere trigono, 

 faciei interna angustissima externa biangulata, — verticali. 



So few species are known of this genus, that it is not necessary to 

 be minute in the specific character. This is by far the largest known. 



