236 



NORTH AMERICAN INDEX FOSSILS. 



4. S. angulatus Hall. Devonic. 

 Tube with two or more volutions, the outer robust; sides sub- 

 angular ; upper angular surface sometimes nodose ; aperture round 

 or oval, usually nearly at right angles to the plane of the spiral. 



Hamilton of New York. 



5. S. arkonensis Nicholson. (Fig. 1518, &, c.) Devonic. 

 Minute ; sinistral or dextral, of two whorls, rounded and some- 

 what globular; last whorl elevated and large; aperture circular; 

 surface with very fine, close-set, thread-like, transverse striae. 



On corals and brachiopods, Hamilton shales of Ontario and 

 western New York. 



6. S. omphalodes Goldfuss. (Fig. 1518, a.) 



Devonic. 



Fig. 15 18. a, Spirorbis omphalodes , vidX. 

 size and enlarged ; b, c^ S. arkonensis, nat. 

 size and enlarged, a sinistral and a dextral 

 shell. (After Nicholson.) 



Fig. 1519. a (left),' Spirorbis 

 annulatus ; b (right), S. nodulosus. 

 Both enlarged. (After Whitfield.) 



Somewhat larger than preceding; surface smooth. 

 Hamilton of New York and Ontario; also Europe. 



7. S. annulatus Hall. (Fig. 1519, a.) Mississippic. 

 Irregularly planospiral, with sharp annulations, with finer ones 



between. 



St. Louis (Spergen) of Indiana and Ilhnois. 



8. S. nodulosus Hall. (Fig. 1519, &.) Mississippic. 

 Volutions strongly deflected, subangular, with obHque ridges or 



striae which become strongly nodose on umbilical side. 

 St. Louis of Indiana and Illinois. 



9. S. anthracosia Whitfield. (Fig. 1520.) Carbonic. 

 Rather high-spired; volutions angular near suture and irregu- 

 larly noded; surface only with coarse growth lines. 



Coal Measures of Ohio, etc. 



