OSTEA.CODA. 651 



Primltla, miuutisslma.l 



broad frill which projects greatly beyond the free contact edges of the valves in 

 species of that genus. As usual, the original conception of Primitia was altogether 

 too broad, and as, through the restless efforts of collectors, the species began to 

 multiply, it became clear that they fell naturally into several groups, whose import- 

 ance increased with time and study till their separation became, at first desirable, 

 then necessary. 



Still, Primitia retains a large number of species, the greater part of which are 

 nearly equally divided between the Lower and Upper Silurian rocks. Two or three 

 rather doubtful species have been described from primordial strata, but at least five 

 good Devonian species have been discovered and as many more in the Lower Car- 

 boniferous, after which the genus seems to have become extinct. With a few exceptions 

 all these species were described in papers by Jones, Jones and Holl, Krause, and 

 Ulrich. 



Pkimitia minutissima, n. sp. 



PLATE XLV, PIG, 31. 



Size.— Length 0.33 mm.; hight 0.19 mm. 



• 



Carapace very small, rather elongate-elliptical in outline, without distinct dorsal 

 angles, the ends rounded and nearly equal, the anterior slightly narrower than the 

 other; valves rather strongly convex; sulcus narrow, sharply defined, extending 

 nearly half across the valve; surface smooth. 



This is the smallest Primitia known to me. It is evidently related to the British 

 Wenloch species, P. humilis Jones and Holl, but is smaller, relatively more convex, 

 with the ends more rounded, and the sulcus narrower. It is not very closely related 

 to any of the known American species. 



Formation and loeality. — Lower part of the Trentoa shales, near Fountain, and at Oxford Mills, 

 Groodhue county, Minnesota. 



Primitia uphami, n. sp. 



PLATE XLIII, PIG. 66. 



Size.— Length 0.42 mm.; hight 0. 27 mm.; thickness 0. 15 mm. 



Valves small, compressed-convex, slightly oblique, subovate, without distinct 

 dorsal angles; posterior end wider and more broadly rounded than the anterior; 

 ventral margin convex; edges thin, without border; sulcus represented by a rather 

 large, though not very deep depression, situated about in the middle of the dorsal 

 slope; surface marked by small punctse, arranged in curved lines radiating from the 

 sulcus; in certain lights each row appears as occupying the bottom of a narrow 

 groove. 



