MOLLUSCA OF LOWER CANADA. 109 



slightly curved : cardinal teeth double, very small, of the same 

 size ; lateral teeth larger, not very prominent. 



Long. 7-16 ; lat. 5-16 ; diameter 4-46 inches. 



Compared to the Sphcerium solidulum, this species is smaller, 

 more inequilateral, less tumid, more compressed, less solid, less 

 heavily sulcated, and its posterior extremity is more distended. 



A very common species in the rivers of Lower Canada ; but 

 appears to have been generally overlooked. 



Figure 4. 

 Sphozrium rhomboideum, Say. (sp.) 



Animal ; white ? syphons reddish-yellow. 



Shell sub-globular, rhombic, orbicular, equilateral ; anterior margin 

 truncated ; posterior slightly angular ; basal nearly straight ; beaks 

 full, but not prominent ; valves slight, convex towards the beaks, 

 gradually decreasing in fullness towards the margins ; interior 

 blue ; sulcations very delicate ; epidermis olive-green, often with a 

 straw coloured zone on the margins ; young shell more compres- 

 sed than the adult ; hinge margin nearly straight ; cardinal teeth 

 rudimentary ; lateral teeth distinct, somewhat acute, not elonga- 

 ted. 



Long. 8-16 ; lat. 6-16 ; diam. 5-16 inches. 



A very local, but gregarious species. 



Figure 5. 

 Sphcerium occidentale, Prime. 



Animal not observed. 



Shell oval, small, pellucid, fragile, equilateral, margins rounded ; 

 valves slight, rather convex ; beaks full, rounded, not much raised ; 

 sulcations very fine, hardly visible ; epidermis horn coloured ; 

 cardinal teeth very diminutive, lateral teeth more distinct. 



Long. 5-16 ; lat. 4-16 ; diam. 3-16 inches. 



