OSTRACODA. 655 



Primltia tumlduiai] 



PrIMITIA TUMIDtJLA, W. Sp. 

 PLATE XLIII, FIGS. 62-65. 



Size.— Length 0.73 mm.; hight0.50 mm.; thickness 0.40 mm. 



Valves strongly convex, subquadrate-ovate, back straight, rather long; posterior 

 dorsal angle strong, the anterior more obtuse or rounded; ends nearly equal in hight, 

 but the anterior margin is more curved than the posterior, the latter being some- 

 what truncated above; sulcus a little in front of, or quite in the middle of the dorsal 

 half, deep, with a strong rounded swelling on each side; the posterior prominence 

 larger and higher than the anterior one, but the latter usually somewhat better 

 defined by a forward swing of the lower part of the sulcus; surface beneath the 

 sulcus prominently convex, and sometimes bearing several small tubercles; a wide 

 concave border, defi^ned in the ventral part by a thin ridge, extending parallel with 

 and some distance within the edge of the valves. This ridge I consider as the 

 remnant of a false border, like the one which is so strongly developed in P. duplicata. 



At first I thought this species might be the same as P. cincinnatiensis Miller sp., 

 but a more careful examination proved it distinct, though perhaps closely related. 

 In the first place its valves are higher, the ventral outline being much more curved. 

 Next, the sulcus is relatively shorter, while the border is not narrow and flat. But 

 the most ipiportant difference is the submarginal ridge which is distinguishable 

 even on casts of the interior of P. tumidula, but of which not a sign is to be seen on 

 Miller's species. In P. duplicata this ridge is much more strongly developed, forming 

 a false border from one dorsal angle to the other. This fact causes the surface of 

 the valves to appear much less convex than it really is, though the greatest thick- 

 ness is a little less than in P. tumidula. But the sulcus in the latter is much deeper, 

 and the tumidity of the surrounding parts greater than in the Trenton species. 



Formation and locality.— In a thin bed of shale belonging to the lower part of the Hudson River 

 group, three miles north of Spring Valley, Minnesota. 



Primitia gibbera, n. sp. 



PLATE XLIII, FIGS. 57-59. 



Size.— Length 0.81 mm.; hight 0.45 mm.; thickness 0.36 mm. 



Valves somewhat leperditoid in outline, with a straight hinge line, the distance 

 between the dorsal angles about five-ninths of the greatest length of the carapace; 

 ends rounded; valves^ rather strongly convex, gibbous in the anterior half of the 

 dorsal region; this prominent part is somewhat flattened on the back, and includes 

 a short and rather shallow notch or sulcus. In the specimens at hand the surface 

 slopes uniformly toward the edges and these seem to be simple and without a border; 



