398 THE PALEONTOLOGY OF MINNESOTA. 



[.Strophomena scofleldi. 



beautifully cancellated by fine, apparently squamose striae, which are undulated 

 slightly upward in passing over the ridges. There appear to be from ten to twelve 

 concentric striae in the width of one line. 



"Width of largest specimen collected, one inch; length, nine lines; hight of 

 ventral area, one line." 



Strophmena recta Conrad we regard as founded on young specimens of the same 

 author's Strophomena deftecta, which is no longer referred to Strophomena, but belongs 

 to Prof. Hall's subgenus Dinorthis of Orthis. Even if the above conclusion is not 

 accepted the specimens of Billings cannot be retained under Conrad's name, since 

 they clearly belong to Strophomena, while Strophomena recta Conrad must be referred 

 to Dinorthis. This leaves Billings' species without a name and we propose therefore 

 to designate it as above. 



S. billinasi belongs to our group II of Strophomena and is related to S. scofaldi. 

 The former differs in having a far less defined sinus and fold, finer radiating striae 

 and the concentric growth lines more delicate and closely crowded. 



Formation and locality. In the Galena shales at St. Paul, near Cannon Falls and Fountain, Min- 

 nesota. In the upper beds of the Trenton limestone, Ottawa, Canada. 



Collectors. W, H. Scotteld and the writers. 

 Mus. Beg. No. 8192. 



STROPHOMENA SCOFIELDI W. and S. 



PLATE XXXI, FIGS. 1821. 



1892, April 1. Strophomena seofieldi W. and S. American Geologist, vol. ix, p. 286. 

 1892, April 9. Streptorhynchus subsulcatum SARDESON. Bulletin of the Minnesota Academy of 



Natural Sciences, vol. iii, p. 335, pi. iv, flg. 39. 



Shell small, semicircular in outline, biconvex, with a more or less prominent 

 fold and sinus towards the anterior margin; hinge-line a little shorter than the 

 greatest width; area of ventral valve forming an angle of about 140 with the 

 plane of the lateral margin, centrally occupied by a convex, perforated deltidium, 

 which fits closely against the chilidium of the other valve. Surface marked by 

 numerous, crowded, rounded, radiating striae, increasing in number by implantation, 

 with from 110 to 120 along the outer margin in adult shells, crossed by delicate, 

 crowded, concentric lines and a few larger growth marks. 



Dorsal valve not deep, evenly convex, or with a fold near the anterior margin. 

 Cardinal area very narrow and slightly reflexed. Crural plates prominent, very 

 oblique, coalescing medially; upon this thickening at its base originate two low 

 ridges, which continue upward and outward into the small, low cardinal process, 

 about half of which is covered by the chilidium. Immediately underneath the 

 crural plates are two pairs of small adductor scars, separated by a low, rounded 



