38 CANADIAN FOSSILS. 



each impression. A thickened ridge lies on the inner side of each, 

 strongest behind the straight-edged anterior impression. 



Fig. 4 shews the variety with the flat or vertical posterior slope ; 

 fig. 5 has it slightly convex. 



Locality. — Plentiful in the Allumetfce limestones. 



C GIBBERULA, N. sp. 



Plate VIII. Figure 6. 



C. subtrigona, rudis, 9 lineas lata, margine antico cardinali gibbo ; latere 

 antico magno, convexo, lunula nulla ; jfostico brevi subcarinato ; cardine 

 dcntibus modicis. 



Differs from the last by characters not very obvious at first sight, 

 but these grow more evident by study, and it seems to bear the same 

 relation to C. contracta that the C. obtusa does to C. Logani. The form 

 is trapezoidal rather than triangular, and of unequal sides; the 

 posterior bluntly pointed and small ; the anterior large, gibbous and 

 rounded ; the ventral margin almost straight. The beak is conside- 

 rably nearer the posterior side. It is not very prominent, the valves 

 being most convex all along the anterior slope which overhangs the 

 hinge-margin all along ; the sinus which separates the somewhat 

 pointed posterior side falls under the beak, and consequently near to 

 that margin ; and the posterior slope is bluntly carinate, and so short 

 as to be not far from vertical. The ligament fulcra are marked out 

 by a narrow lunette, which the beak overhangs. 



The hinge-plate is bent at an obtuse angle, and bears about ten 

 teeth on each side, which are set obliquely, as in the last species. 

 The shell is thick, but the muscular impressions are scarcely visible 

 in our specimens. A few antiquated lines of growth near the margin 

 shew that our specimen is full-grown. 



Notwithstanding the above important differences, there is much 

 similarity to the last species in habit, the distinction being chiefly 

 due to the exaggerated development of the anterior side. 



Locality. — Not uncommon in the Allumette limestone. 



