654 THE PALEONTOLOGY OF MINNESOTA. 
[Primitia duplicata. 
projecting rim which however is not distinguishable in a side view from the regular 
slope of the surface except in front; sulcus sharply defined, simple, subcentral, 
extending less than one-third of the distance across the valve; surface minutely 
punctate. 
The widely grooved edges distinguish this species from several otherwise similar 
forms occurring in the Upper Silurian of Europe. The projecting rim, which should 
not be mistaken for an ordinary border, is to be regarded as an undeveloped “ frill” 
and precisely the same as the false border of Ctenobolbina ciliata and Ceratopsis 
chambersi. It is developed to a greater degree in the next species, but in P. tumidula 
it appears to have been in a large measure reabsorbed again. 
Formation and locality.—Middle third of the Trenton shales, Minneapolis, Minnesota. 
PRIMITIA DUPLICATA, #1. Sp. 
PLATE XLITI, FIGS. 60 and 61. 
SizzE.—Length 0.70 mm.; hight 0.45 mm.; thickness 0.35 mm. 
Valves rounded oblong-quadrate, with a long, straight back, rounded dorsal 
angles, and a distinctly elevated false border. This border projects slightly beyond 
and completely hides, in a side view, the anterior and ventral contact margins of 
the valve. Posteriorly however the true edge protrudes, the border here projecting 
outwardly much more than backward. Within the border the surface is moderately 
convex, the sulcus not deep yet distinct, and faintly traceable for about two-fifths 
the hight of the valve. In front of the lower part of the sulcus a small swelling is 
faintly indicated, while behind its upper two-thirds there is another but much larger 
low elevation. Surface without ornamentation so far as known. 
This interesting species agrees with P. celata in having a false border, but as it 
is more elevated, especially in its posterior part, and as the two formsare quite different 
in the region of the sulcus, it is not at all likely that they will ever be confused by 
a careful observer. I know of no form now referred to Primitia, unless it be P. 
tumidula, which see, that is sufficiently near P. duplicata to require comparison. 
Beyrichia initialis, an associated species, looks considerably like it. It is of about 
the same size, and hasaraised border. A critical examination of the latter however 
proves that it is not a false border, but the actual margin of the valve bent outward 
(compare figs. 61 and 83, plate xi). Of course the lobing of the valves, though 
certain similarities may be discovered, is still very different in the two forms. A 
comparison of their respective figures on plate xt will bring out the differences 
much better than I can define them, 
Formation and locality—Middle third of the Trenton shales, Minneapolis, Minnesota. 
