648 Tjjj; PALEONTOLOGY OP MINNESOTA. 



[Prlmlblella llmbata. 



gently convex, or almost straight in the middle; posterior margin somewhat oblique, 

 and subtruncate above; anterior outline always more curved than the posterior; free 

 edges with a narrow border; surface with abroad, centro-dorsal depression. 



The earliest known occurrence of this species is in the lower part of the Birds- 

 eye limestone of Kentucky. These specimens differ slightly from the later form in 

 having the border much narrower, the ventral margin straighter and quite parallel 

 with the dorsal, and the anterior outline more rounded. The valves seem also to 

 be a little more convex. 



Primitiella elongata, var. nuda Jones,* is similar' but has straighter ends and 

 sharper dorsal angles. 



Formation and locality.— LiOVfer. and upper Birdseye limestone, High Bridge, Kentucky, and Leba- 

 non and Lavergne, Tennessee; lower third of the Trenton shales, Minneapolis, St. Paul, and Goodhue 

 county, Minnesota. 



Pkimitiella limbata, n. sp. 



PLA.TE XLIII, FIGS. 53—56. 



Size.— (E. C.) Length 0.73 mm.; hight 0.38 mm.; thickness 0.20 mm. 



The outline is almost as in P. constricta, only the ends are less rounded, the 

 posterior one especially being nearly vertical, while the dorsal angles are sharper. 

 The most important difference however lies in the fact that the border continues 

 not only around the free edges but along the dorsal margin as well. The thickness 

 of the carapace is somewhat less, and the surface rises more abruptly from the 

 posterior border. Finally, the mesial depression is more obscure, and often scarcely 

 distinguishable. 



Formation and locality. — Lower third of the Trenton shales, Minneapolis, Minnesota. 



Primitiella simulans, n. sp. 



PLATE XLIII, FIGS. 26-28. 



Size. — Length 0.73 mm.; hight 0.44 mm.; thickness 0. 28 mm. 



Valves rather strongly convex, leperditoid in outline, with the dorsal angles 

 rounded; edges without border; a very faint, broad depression near the middle of 

 the dorsal slope, and occasionally an obscure elevation at its base. 



In the outline this species is very nearly like P. minuta Ei"chwald and Aparchites 

 subovatus and leperditoides Jones. Still it is relatively higher than any of these, and 

 the last two are without the dorsal depression. It resembles also A. minutissimus 

 Hall, but may be distinguished by its proportionally greater length. 



Formation and locality.— Lower part of the Trenton shales, near Fountain, Minnesota. 



* Prol. Jones describes this form as a variety of Primitia elongata Krause, but since Dr. Krause has shown that his 

 species possess a radially striated false border lilce that of EurycMHnaf aubcequceta, Ulrich, the form nuda should now be 

 regarded as at least speoifloally and probably generlcally distinct from P elongata. 



