434 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



very weak. Some individuals have one lateral plication on the fold 

 and one in the sinus fairly well developed, while those oji the opposite 

 side merely appear or are entirely absent. The loss of these plica- 

 cations is due to obsolescence. Among the specimens in Columbia 

 Universitj' there were several which showed this loss to be due to 

 retardation in development, but none of the s]^ecimens belonging to 

 the National Museum show this and the Columbia specimens are not 

 regarded as representative. 



Measurements. — 28.5 mm. along the hinge line, 24 ram. wide across 

 the middle, 18.3 mm. high, 22 mm. thick, 13 mm. sinal width, 16 mm. 

 smal depth, 6 mm. fold depth. 



Occurrence.-— M&ysviWe Group. Bellevue: Madison aiul Vevay, 

 Indiana; Maysville: Near Lebanon, Kentucky. Corryville: Cincin- 

 nati, Ohio. 



Plesiotyfes.—Q'At. Nos. 39037, 65968, 65969, 65970, U.S.N.M. 



PLATYSTBOPHIA UNICOSTATA CRASSIFORMIS, new variety. 

 Plate 48, ti<?s. 8-10. 



This variety is represented by narrow gibbous s])ecimens which 

 show strong lateral compression. The hinge line equals or is greater 

 than the width across the middle. The fold is narrow and strongly 

 elevated; the sinus is profound. In most s]>ecimens there are two 

 plications on the fold and one in the sinus, but in other specimens 

 there is one weak lateral plication on one or both sides of the fold 

 and sinus. The lateral slopes of the brachial valve are strongly 

 convex, those of the pedicle valve strongly concave. The slopes are 

 occupied by four to six plications. 



This variety has been confused with PJatystrophia crassa. It has 

 a more angular outline, the fold is higher, the sinus deeper, and the 

 lateral plications are broader and more broadly spaced. It is nar- 

 rower than PlatystropMn vAiicostata. The many characteristics in 

 common between this variety and Platystrophio cypha would indicate 

 that it belonged to that species, but the growth stages and many 

 variants point to Platystrophia laticosto as the stock from which this 

 variety developed. 



Measurements. — 25 mm. along the hinge line, 22.2 mm. wide across 

 the middle, 17.1 mm. high, 19 mm. thick, 11.5 mm. sinal width, 11,5 

 mm, sinal depth, 4.5 mm. fold depth. 



Occurrence. — Maysville grouji. Leipers formation; Boodlettsville, 

 Tennessee. Bellevue member: Maysville, Kentucky: Bellevue; 

 Madison, Indiana. 



Cotypes.—C-At. No. 6.5965, U,S.N,M. 



