Silurian and Devonian dossils of Canada. 435 



regularly rounded ; a portion of the ventral margin about the cen- 

 tre of the width is sometimes nearly straight; dorsal margin ele- 

 vated, somewhat compressed ; diagonally and rounded ventricose 

 from the umhones towards the posterior ventral angle ; beaks short, 

 obtusely rounded, incurved ; surface nearly smooth or obscurely 

 marked with concentric ridges ; a few strong imbricating lamellae 

 of growth near the margin of some specimens. Width from fifteen 

 lines to two inches and one-fourth ; length from eleven lines to 

 twenty-one lines. 



Some of the specimens are a little more transverse than others ; 

 but there are intermediate forms connecting the specimen, repre- 

 sented by Figure 9, with Figures 8 and 10. 



Fig. 11. 

 Fig. 11. A fragment, shewing the anterior teeth. 



The anterior teeth are short, the central one being the longest 

 and the most curved ; the posterior teeth of the specimen repre- 

 sented by Fig. 10 are two in number, elongated and prominent. 



Locality and Formation. — Island of St Joseph's Lake Huron ; 

 La Petite Chaudiere Rapids near the City of Ottawa; Fourth 

 Chute of the Bonne-chere and Pauquette's Pvapids ; associated 

 with fossils of the Trenton and Black River formations. 



Collectors — Sir W. E. Logan, J. Richardson, A. Murray, E. Bil- 

 lings. 



Cfrtodonta spinifera (Billings). 



Fig. 12. 



Descrijrtion. — Small, sub-circular ; greatest length and breadth 

 about equal; moderately convex ; hinge line much elevated ; ura- 

 bones small, incurved ; dorsal margin nearly straight from the urn- 



