350 THE PALEONTOLOGY OF MINNESOTA. 



[Lingula iowensis. 



description, there can be no doubt that the name L. iowensis was applied by him to 

 the widely distributed and characteristic Lingula of the Galena horizon. Professor 

 Whitfield was the first to make use of Owen's name; the following is a copy of his 

 description, with our additions: "Shell large, broadly ovate, elliptical or subquad- 

 rate in outline, generally a little narrower above the middle of the length than 

 below ; upper end very obtusely angular, the cardinal slopes forming an angle with 

 each other of about one hundred and twenty degrees ; sides of the shell gently 

 rounded, and the basal line more sharply rounded, but never truncate. Valves 

 convex, the ventral most strongly so, and generally subangular along the middle. 

 Ventral beak projecting a short distance beyond the dorsal, and more pointed. The 

 cardinal margins of the ventral [as well as the dorsal] valve are infolded along 

 their border, forming an imperfect cardinal area of a very perceptible width on 

 well-preserved specimens." These areas are striated, and immediately below the 

 apex of each valve, the arched striae meet, forming in the ventral valve a very 

 slight linear elevation. 



Gutta-percha impressions made from natural casts of the interior of the dorsal 

 valve show a pronounced median septum, which is but slightly developed near 

 the posterior end of the valve, and extends somewhat beyond the center of the 

 shell, where it is strongly elevated, and terminates abruptly. On each side of the 

 septum, for two-thirds of its length, are well-defined, progressive, concrete lateral, 

 and central muscular scars. In the ventral valve, the median septum is faintly 

 indicated, and does not extend beyond the progressive, lateral muscular scars, 

 which are one-third the entire length of the valve. In front of these impressions, 

 at the anterior end of the septum, are the small, middle lateral scars, which extend 

 somewhat beyond the central muscular imprints on each side. 



" Surface of the shell marked by strong, irregular lines of growth at irregular 

 distances, the outer margins of which are slightly raised and free, presenting a 

 strongly lamellose appearance under a magnifier ; the spaces between being smooth 

 and often polished. On exfoliated specimens, and more distinctly on internal casts, 

 the surface is very strongly radiated by fine flattened, but irregular striaB for from 

 one-third to one-half the length of the shell, and on nearly the entire width along 

 the basal line ; but a little higher on the sides they lose their regularity and become 

 broken and wrinkled as well as more strongly divergent." The striae are confined 

 to the internal surface of the valves. 



Lingula cincinnatiensis Hall and Whitfield is a closely related species, and may 

 be a direct descendant from L. iowensis Owen. It differs from the latter in being 

 usually more robust,' the valves deeper or more convex, and in having the posterior 

 portion of the shell more pointed. 



