FOSSILS OF TIIK r()AL MKASriIKS. 601 



PLEUROTOMAKIA SPIBONEMA, M. and W. 



PI. 28. Fie 5. 

 PletirotC'Tnaria gpiranema, MEEK and WORTHEN, 1866. Proceed. A cad. Nat. Sci., Phila., p. -27-J. 



SHELL under medium si/e, sub-globose, with length and 

 breadth nearly equal. Volutions live or six, increasing 

 rather rapidly in size; those of the spire, convex; the last 

 one forming more than four-fifths of the entire length, and 

 so murh as nine-tenths the entire bulk of the shell, rounded 

 regularly from the suture above to the umbilicus below, 

 excepting near the aperture, where it is a little more prom- 

 inent below than above the middle. Aperture sub-orbicu- 

 lar in general outline, but rather strongly modified above 

 the middle on the inner side, by the return of the body 

 whorl. Inner lip slightly thickened and deeply arcuate 

 below, but wanting or exceedingly thin above the middle 

 of the aperture; coluuiella tortuous, with a slightly im- 

 pressed furrow at the outer margin of the inner lip, but 

 without an umbilical perforation. Surface ornamented 

 with regular, distinct, revolving stria?, crossed just below 

 the suture by short, little node-like folds, confined to the 

 narrow space between the suture and the spiral band; simi- 

 lar but smaller, more crowded and longer curved wrinkles 

 also radiate from the umbilical region on the under side of 

 the body whorl. Lines of growth obscure on all the speci- 

 mens seen. Spiral band flattened so as to be even with the 

 general surface, nearly smooth, and placed half way be- 

 tween the middle of the body whorl and the suture above, 

 or about once and a half its own breadth below the suture. 



Length and breadth of a medium sized specimen, each 

 0.45 inch; length of aperture, 0.25 inch; breadth of do., 

 0.23 inch; apical angle, with convex slopes, divergence, 

 90 degs. ; breadth of spiral band at the aperture, 0.07 inch. 



This species is nearly related to P. Beckiritlmna, of McChesney (Xew 

 Paheozoic Fossils, p. (>l,) with which we supposed it to be identical, 



