650 THE PALEONTOLOGY OF MINNESOTA. 
[Primitia. 
his drawings (op. cit., pl. rv, figs. 8—13), witn the figure here given on plate xxi, 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 
are from Bala shales, near Welshpool, Montgomeryshire. 
Genus PRIMITIA, Jones and Holl. 
Primitia (part.) JONES and Hout, 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 
suleus; 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 H., Upper Silurian; P. impressa Ulrich, 
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, Primitia included (1) “non-sulcate” forms for which Jones in 1889, pro- 
posed the genus Aparchites; (2) so-called “passage forms” that I now propose to 
separate as Primitiella; (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 
