2 NORTH AMERICAN INDEX FOSSILS. 



f. Shell not made up of concentric cones *. 



^. Star-shaped muscle marking present on inside of operculum 



IV. Hyolithellus 



*. No star-shaped muscle marking present \^ 



l^. Transverse striae more or less oblique VI. Coleolus 



\^ . Transverse striae straight II. Orthotheca. 



2. Cross section usually triangular, sometimes oval I. Hyolithes. 



2. Cross section elliptical, tube strongly curved III. Helenia. 



Family Hyolithid^ Nicholson. 

 I. Hyolithes Eichwald. 



Shell, of calcium carbonate, straight or curved; cross section 

 triangular or elliptical. The margin of the flattened dorsal side 

 projects somewhat above the opposite wall causing thus a forward 



P'iG. 121 1, a-d, Hyolithes am eric anus ; Cy section; </, operculum ; e,f H. billingsi ; 

 g, operculum; h-j\ H. impar; k, operculum; /-w, H. similis ; o-q^ H. communis; 

 r, operculum. ( After Walcott. ) 



