432 



Silurian and Devonian Fossils of Canada. 

 Cyrtodonta rugosa (Billings). 



Fig. 1. 



Fig. 2. 



Figure 1. Exterior of right yalve. 

 " 2. Interior of same specimen. 



Description. — Small, sub-rliomboidal or sub-quadrate, the dor- 

 sal and ventral margins being somewhat parallel, and the anterio r 

 and posterior extremities obtusely rounded, the latter broader than 

 the former ; obliquely tumid from the beaks to the posterior ven- 

 tral angle ; the beaks rather small and incurved ; a broad, shallow, 

 scarcely perceptible depression extending from the ventral margin 

 obliquely forward and upward towards the umbones ; surface con- 

 centrically striated, and also marked with several more or less pro- 

 minent sub-imbricating concentric ridges of growth ; hinge line 

 nearly straight, a little curved ; interior shewing in the right valve 

 three anterior teeth, the central one of which is the largest • 

 two posterior lateral teeth, In the left valve there appear to be 

 four anterior teeth ; but as the specimens are somewhat imper- 

 fect, this may not be the correct number. Width nine lines ; 

 length from the centre of the hinge line to the centre of the ven- 

 tral margin, seven lines ; depth of a single valve, three lines. 



None of the specimens that I have seen are larger than the one, 

 represented in figures 1 and 2. 



Locality and Formation. — Fourth Chute of the Bonne chere 

 Pauquette's Rapids, and La Petite Chaudiere Rapids near the city 

 of Ottawa north side, associated with numerous fossils of the Tren- 

 ton and Black River formations. 



Collectors — Sir W. E. Logan, J. Richardson, E Billings. 



Cyrtodqjstta Huronensis (Billings). 



Fig. 3. Fig. 4. 



Figure 3. View of left valve from Lake Huron. 



" 4. Interior of another specimen, same locality. 



