340 NORTH AMERICAN INDEX FOSSILS. 



blunt. Valves unequal, the right the larger and overlapping the 

 ventral edge of the left; hinge simple. Surface frequently horny 

 in appearance, smooth and glossy in most cases, granulose or 

 minutely punctate in others ; a small tubercle or " eye-spot " is gen- 

 erally present on the anterodorsal fourth, while a large rounded, 

 subcentrally situated muscular imprint is a well-marked feature 

 of the interior and sometimes distinguishable even on the exterior. 

 Genotype L. brittanica Rouault. 



About forty species. Range : Chiefly Cambric, Ordovicic and 

 Siluric, the Devonic and Cambric species being small and doubt- 

 fully referred to the genus. 



6. L. fabulites Conrad. (Fig. 1653.) Ordovicic. 

 Of medium size; greatest length about the middle of shell; 



anterior end more sharply rounded than posterior; eye tubercle 

 small; hinge line central. 



Stones River of Minnesota, Cincinnati region, Tennessee, New 

 York and Pennsylvania; Black River of Canada. 



7. L. caecigena S. A. Miller. (Fig. 16^6, d,e.) Ordovicic. 

 Anterior end less produced than posterior ; anterobasal margin 



sloping abruptly. 



Cincinnati group of Indiana. 



8. L. angulifera Whitfield. (Fig. 1654.) Siluric. 



Fig. 1654. Leperditia angulifera right, left and anterior views. ( After Whitfield. ) 



Anterior end nearly rectangular; sides flat, abruptly inturned 

 basally and anteriorly. 



Lower Monroe of Ohio, etc. 



9. L. scalaris Jones. (Fig. 1655.) Siluric. 



Large general form similar to L. fabulites, but left valve with 

 strong rounded and elongate node below hinge line. 



Cobleskill of eastern New York, New Jersey, etc. ; Akron of 

 western New York, Ontario, etc. 



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