New Species of Palaeozoic Fossils. 185 



form, and sometimes becoming obscurely trilobate. Dorsal valve 

 less convex than the ventral, and varying from depressed-convex to 

 ventricose ; in some conditions having a mesial elevation, becoming 

 angular along the cardinal slope with the margins abruptly incurved ; 

 the incurved spaces usually free from plications ; beak closely 

 incurved into the triangular fissure of the opposite valve. Yentral 

 valve more convex, varying from depressed-convex to ventricose, 

 and strongly arcuate in older shells ; with or without a mesial fold, 

 which becomes developed with age and produced in front ; beak in 

 young shells scarcely incurved, becoming more arcuate with age, 

 projecting above and beyond the umbo of the dorsal valve, and 

 showing beneath it the upper part of the triangular fissure. Cardinal 

 slopes, especially in the older shells, abruptly incurved and concave. 

 Surface marked by distinct and well-defined, simple, rounded or 

 subangular radii, which vary in strength at different stages of 

 growth and in shells of the same size, the number ranging from 

 twenty-five to forty in shells of medium size. In the perfect con- 

 dition the surface is marked by fine concentric striae and with distinct 

 rugose lines or lamellae of growth. 



In the specimens examined (fourteen in number), there is a great 

 variety of aspect and of surface marking. The smallest specimen is 

 about four lines in length, with a somewhat greater width, and a 

 depth of a little more than two lines ; while the largest specimen 

 has a length of two and a quarter inches, a width of a little more 

 than one and three-quarters, and a depth of nearly one inch and a 

 half. Some of the younger shells are marked with fine and some 

 with coarse radii. One specimen of intermediate size has a length 

 of scarcely an inch, a breadth of one inch and two lines, and a depth 

 of six-tenths of an inch ; the surface is marked by forty-one radii. 

 Another specimen has a length of one inch and four-tenths, a width 

 of little more than one inch and two-tenths, and a depth of seven- 

 tenths of an inch ; the surface is marked by twenty-five strong radii. 



Two distinct varieties may be recognized ; the one having coarse 

 and the other with finer radii, which may be designated as P. JVysius 

 var. crassicosta and P. JVysius var. tenuicosta. 



In form and surface markings this species bears some resemblance 

 to P. multicostatics H., but the latter is proportionally more gibbous 

 in the umbonal region and slopes more abruptly to the front, with 

 no indication of mesial fold or extension, but rather a faint depression 

 with truncate anterior margin. 



Formation and locality. In beds of the age of the Niagara group, 

 near Louisville, Ky. Cabinet of Dr. James Knapp. 



