NO. 2297. BRACHWPOD GENUS FLATYSTROFHIA—McEWAN. 437 



Compared with Platystrophia waUowayi this form has more acum- 

 mate cardmal extremes with concave rather than convex lateral mar- 

 gins. The slopes of the brachial valve are more tumid and those of 

 the pedicle valve more compressed than in the latter species. It has 

 many of the characteristics of Platystrophia unicostata. The slopes 

 of the brachial valve are more swollen and those of the pedicle valve 

 more compressed than that form and it also has a greater number of 

 plications on the slopes. 



The inflated brachial valve suggests relationship to Platystrophia 

 clerTcsvillensis and Platystrophia cumingsi and the stratigi'aphic 

 position of the three forms strengthens this view, but this variety- 

 marks the culmination of a development toward decrease in width 

 and increase in thickness. It would not be likely to give rise to 

 species whose development was toward increase in the width of the 

 sheU. 



Measurements. — 33.5 mm. along the hinge line, 25.9 mm. wide 

 across the middle, 19.3 mm. high, 25 mm. thick, 16.8 mm. sinal 

 width, 16 mm. smal depth, 6.5 mm. fold height. 



Occurrence. — Richmond group: Ai-nheim formation: One mile 

 south of Mount Washmgton, Greenwell Ford, li miles south of 

 Clark, etc., Kentucky; 4 miles northeast of Gallatin and Goodletts- 

 viEe, Tennessee; WaynesviUe and ClarksviUe, Ohio; Southeastern 

 Indiana. 



Holotype.— Cat. No. 66069, U.S.N.M. 



PLATYSTROPHIA CYPHA ARCTA, new variety. 



Plate 48, figs. 1-3. 



Another variant of Platystrophia cypTia has a thinner valve and a 

 more compressed fold and sinus than other members of the species 

 and the lateral plications on the slopes of the fold and sinus are weak. 

 Sometunes they become obsolete before reaching the frontal margin. 

 Most of the specimens have subquadrate cardinal extremes; a few 

 specimens are slightly elogate on the hinge-line. The convexity of the 

 brachial valve and concavity of the pedicle valve are not so marked as 

 in other members of the species. Nine and ten closely spaced plica- 

 tions occupy the lateral slopes. 



This variety is easily distinguished by its thin fold, nairow form, and 

 closely spaced plications. 



Measurements. — 35 mm. along the hinge line, 33 mm. wide across 

 the middle, 17.5 mm. high, 13.5 mm. thick, 12 mm. sinal width, 

 9.5 mm. sinal depth, 6 mm. fold depth. 



Occurrence. — Richmond group. Arnheim formation: Wyoming, 

 Sunset, i mile east of FisheiviUe, Clark, Greenwell Ford Section, 

 Bullitt County, Kentucky; Gallatin, Tennessee; WaynesviUe, Ohio. 



Cotypes.— Cat. No. 66045, U.S.N.M. 



