654 THE PALEONTOLOGY OP MINNESOTA. 



[Primltia dupUoata. 



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 Ctenobolhina 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 dtjplicata, n. sp. 



PLATE XLITI, FIGS. 60 and 61. 



Size.— 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 forms are 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 has a raised 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 xliii). 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 xliii will bring out the differences 

 much better than I can define them. 



Formation and locality.— T&i&&\& third of the Trenton shales, Minneapolis, Minnesota. 



