412 PROCEEDINGS OF THE NATIO^^AL MUSEUM. vol. 50. 



Occurrence. — Trenton group: Bigby limestone, Frankfort, Ken- 

 tucky. Trenton limestone, Ellisburg;, New York. 

 Cotypes.—C&t. No. 66124, U.S.N.M. 



PLATYSTROPHIA AMOENA, new species. 



Plate 43, figs. 1-8. 



Platystrophia lynx Cumings, Araer. Journ. Sci., ser. 4, vol. 15, 1003, pp. 20, 22, 24^ 



figs. 8, 10. 

 Orthis biforata Hall and Clarke, Pal. New York, vol. 8, pt. 1, 1892, pi. 5B, fig. 10. 



This is a thin transverse species with subequally convex valves. 

 The hinge line is as long or nearly as long as the width across the 

 middle. The fold is low and spreading and the sinus is shallow. The 

 fold is occupied by four and the sinus by three plications. There are 

 nine plications on the lateral slopes. 



This species resembles Platystrophia elegantula, new species, but it 

 lacks the curved plications on the lateral slopes, and it has a longer 

 hinge line than that species. A much thicker variety is found at 

 various localities associated with typical specimens. The thickness 

 may become as great as or greater than the height. Shells of this 

 type are abundant at Trenton Falls, New York, and for these the 

 name Platystrophia amoena rohusia (pi. 43, figs. 14-17, Cat. No. 66091, 

 U.S.N.M.) is suggested. 



Measurements. — 13 mm. along the hinge line, 14 mm. wide across 

 the middle, 10 mm. high, 6.3 mm. thick, 5.5 mm. sinal width, 2 mm. 

 sinal depth, 0.7 mm. fold depth. 



Occurrence. — -Trenton group. Bigby limestone, Versailles, Frank- 

 fort, Lexington, Benson Station, etc., Kentucky. Trenton limestone 

 of Trenton Falls, New York. Prosser limestone of Cannon Falls and 

 Warsaw, Minnesota. Cynthiana formation, Tennessee. 



Cotypes.—C^i. Nos. 39057, 48612, 48615, 66072, 66076, 66091, 

 U.S.N.M. 



PLATYSTROPHIA AMOENA LONGICARDINALIS, new variety. 



Plate 43, figs. 9-13. 



The distinguishing characteristic of this variety is the great width 

 in reference to the height. The hinge line always forms the greatest 

 width and the cardinal extremes are acuminate and thin. The thin- 

 ness is due to the compression of the slopes of both valves. 



Owing to the long hinge line this variety resembles Platystrophia 

 trentonensis, new species, but the plications of the latter species place 

 it with the biplicate group of shells, while this species belongs to the 

 triplicate group. 



Measurements.— '\b mm. along the hinge, 12 mm. wide across the 

 middle, 8.2 mm. high, 7 mm. thick, 5.5 mm. sinal width, 3.8 mm. 

 sinal depth, 1 mm. fold height. 



