Silurian Fossils. 25 



fully 3 mm. The general outline of the shell is semi-circular, 

 the width being greater near the hinge line than across the 

 middle of the shell. The convexity of the pedicel valve is 

 greatest along the median parts of the shell, producing a semi- 

 globose area in the more convex specimens, beyond which the 

 postero-lateral parts project with a more or less concave out- 

 line when viewed from the side of the hinge area. Radiating 

 striations 10 in a width of 2 mm. at a distance of 3.5 mm. 

 from the beak ; this results usually in about 40 striations 

 within 4 mm. of the beak, although in the more convex spec- 

 imens the number of radiating striations may increase to 50. 

 Concentric striations are fine, close, and not conspicuous even 

 when seen under a lens. A vertical spine one millimeter in 

 length is located about 1.5 mm. from the middle line of the 

 shell on each side of the beak. A slightly longer spine, inclining 

 outward at an angle of 35 to 45 degrees, is located about 

 2.5 mm. from the middle line, on each side of the beak. In 

 one specimen spines were found three and a quarter mm. 

 from the middle line, but it has been impossible so far to 

 distinguish more than 4 spines on the same shell. 



Kokomo limestone at Kokomo, Indiana; common in the 

 upper part of the exposures at the Defenbaugh quarry. 



Compared with Chonetcs cornutus, Chonctcs novascoticns, 

 Chonetes temiistriatus, and Chonetes jerseyeiisis, the distinctly 

 greater width near the hinge line, compared with the width 

 across the middle of the shell, is a distinguishing feature. 

 Moreover, the convexity of the pedicel valve is considerably 

 greater. Compared with Chonetes niidulatus, the convexity 

 is greater, and the radiating striations are much more 

 numerous. 



Platystrophia pauciplicata, sp. nov. 



Plate I, Fig. 15. 



This specimen of Platystrophia is distinguished by the 

 presence of only two plications on the median fold, and three 

 plications on each side of this fold, with a faint indication 



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