156 



CONCHOCELE CRETACBA, WhITEAVES. 



Conchocele cretacea^ Whiteaves. — Geol. Surv. Canada, Rep. Progr. 1873-74, p. 266, pi. of 



foss., figs. 2 and 2a. 



Shell yeiy inequilateral, convex, inflated, especially in the umhonal 

 region ; outline subquadrate, length nearly one fourth greater than the 

 height; test thin. Umbones broad, tumid, anterior and terminal ; beaks 

 slender, subspiral, depressed, their apices curving downwards and a little 

 outwards. Lunuie large, ovate cordate, about one fourth higher than 

 wide ; concavely and shallowly excavated, its margin below being 

 sharply angular, but not very prominent. Escutcheon or cardinal area 

 very distinctly defined, broadly lanceolate as seen from above, and 

 marked by two longitudinal and prominent keels or ridges on each valve. 

 The two outer ones, which together form the outer boundary of the 

 escutcheon, commence at the beaks, curve first outwards, then inwards, 

 and finally meet at the upper termination of the posterior end. The two 

 inner ridges both commence a,t the beaks, after which they each curve 

 gently outwards, and then gradually inwards, until they meet on the 

 cardinal border at a short distance from the posterior end. Their 

 summits are acute, and they are each separated from the outer ridges 

 and from the cardinal border, except at the point where they intersect 

 the latter, by a deeply concave groove. Together they enclose, a 

 narrowly lanceolate and shorter subordinate area within the escutcheon. 

 Anterior margin shallowly concave under the beaks, and forming a 

 subangulai" junction with the upward curve of the basal margin near or 

 a little below the middle. Ventral border broadly rounded, curving 

 upwards most rapidly at the jjosterior end, which is subtruncated and 

 angulai" at its junction with the cardinal margin above. As viewed 

 laterally, the outer ridge which bounds the escutcheon is so prominent as 

 to hide most of the true cardinal margin from view, and in this aspect 

 the outline of the former is slightly convex near the beaks, after which it 

 slopes gently downwards to the upper termination of the shell posteriorly. 

 A portion of the hinge margin, however, projects very slightly above 

 the centre of the highest level of this ridge in the shape of a [depressed 

 but broadly convex lobe. 



Surface marked by fine, subequal, concentric, raised strise. 

 In the left valve the hinge ajjpears to be comjiosed of a single, long, 

 sharp tooth, which runs parallel with the cardinal margin, and which 

 extends from the beaks almost to the posterior end. In the right valve 



