650 THE PALEONTOLOGY OF MINNESOTA. 



[Prlmitia. 



his drawings (op. cit., pi. iv, figs. 8 13), with the figure here given on plate XLIII, it 

 would appear that none of his specimens are strictly identical with the typical form 

 of the species. They are all too narrow anteriorly, and three of the figured ones too 

 long. The other three figures (8, 9 and 10) correspond fairly well with that of the 

 Manitoba specimen already referred to, though the posterior spine in the last is 

 stronger. Possibly some of the variability of the Bala specimens is due to crush, or 

 perhaps their margins were covered by the shale. There remains to be added that 

 in all these foreign specimens the border, as well as the slight elevation in the dorsal 

 depression, seems to be wanting. Under the circumstances it would probably be 

 advisable to separate them, if not specifically, at any rate as a variety, from the 

 typical form of the species. 



Formation and locality. Doubtfully identified from a cast of the interior found in a thin bed of 

 shale belonging near the base of the Hudson River group, three miles north of Spring Valley, Minnesota. 

 The typical form occurs abundantly in the lower or Utica horizon of the Cincinnati group at a number of 

 localities in the vicinity of Cincinnati, Ohio. The Manitoba variety is from beds equivalent to the upper 

 divisions of the Cincinnati group at Stony Mountain, while the .British specimens described by Prof. Jones 

 art- from Bala shales, near Welshpool, Montgomeryshire. 



Genus PRIMITIA, Jones and Holl. 



Primitia (part.) JONES and HOLL, 1865. Ann. and Mag. Nat. Hist., ser. 3, vol. xvi, p. 415. 



Carapace small, varying in outline, usually subovate, but the hinge is always 

 straight; valves equal, never overlapping, generally provided with a narrow border; 

 in, or to one or the other side of, the middle of the dorsal half, a well-marked pit or 

 sulcus; the pit may be rounded and situated subcentrally, or it may be drawn out 

 vertically so as to extend from the dorsal margin half across the valve; on one or 

 both sides of the sulcus the surface may be raised into a low, rounded or ridge-shaped 

 prominence. Surface of valves punctate, reticulate, or without ornament; in rare 

 cases it seems to have been minutely granulose. 



As typical species I will mention P. mundula Jones, P. renulina Jones and Holl, 

 P. variolata J. and H., and P. humilis J. and II., Upper Silurian; P. impressa Ulricb, 

 P. sancti pauli UL, and P. mammata Ulr., Lower Silurian, the last two described in 

 this work. 



Prior to 1865, species of Primitia were referred to Beyrichia. For more than 

 twenty years after that date, besides the type of structure to which the genus is now 

 restricted, Primitiit included (1) "non-sulcate" forms for which Jones in 1889, pro- 

 posed the genus . I/M/V////V.S; (2) so-called "passage forms" that I now propose to 

 separate as I'rh/ii/iella; (3) forms having the sides of the sulcus elevated into two 

 strong tubercles, for which the genus Ulrichia has been established by Prof. Jones; 

 and finally (4) some that may belong to Eurychilina, Ulrich, because they have the 



