134 



cardinal edge ; foramen triangular, its width at the hase ahout equal to 

 the height. Dorsal valve moderately and uniformly convex ; a small 

 portion at the cardinal angles compressed and often slightly reflected ; area 

 a little less than that of the ventral valve, overhanging the hinge-hne ; 

 umbo small, obtusely rounded ; beak forming a small elevation upon, but 

 scarcely distinct from the cardinal edge. 



Surface with sub-angular radiating striae, 5 or 6 lq the width of 2 lines 

 in the middle of the shell, and about 4 at the front margin. These striae 

 are subdivided at about one-third the length from the beak, and again at 

 about two-thirds. 



Width of a large specimen 1 inch ; length | of an inch ; width of the 

 area 6 lines ; height of the same 1 hne. 



It will be seen from the details given in the above description, that this 

 species is most closely allied to 0. subquadrata, and perhaps should be 

 regarded in the light of a variety thereof. There are, however, certain 

 difierences which are very persistent, although not great. In 0. subquadrata 

 the dorsal valve has always a mesial sinus, and the ventral, a mesial fold ; 

 the area is always more than one half the width of the shell ; the most 

 elevated point of the ventral valve is distant from the beak a little more 

 than the height of the area, and the shell is somewhat larger. In this 

 species there is a sinus in the ventral valve, instead of a fold, and the 

 dorsal valve is uniformly convex or exhibits no sinus ; the area is about 

 one-third the whole width, and the greatest elevation of the ventral valve 

 is nearer the beak than it is in 0. subquadrata. This latter species ranges 

 from the Trenton to the Hudson River 'group, but 0. Ipliigenia occurs 

 only in the Trenton. 



Locality and Formation. — City of Ottawa ; Trenton hmestone. 



Collectors. — E. Billings ; A. T. Drummond, Esq., of Kingston. 



