FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 171 



PleUTOtOmaria Casii. MEEK AND WORTHEX. 



Plate XX VI., figure 11. 

 Pleurotomaria cusii, M. and W. 111. Gcol., Vol. 3, p. 3591868. 



Shell attaining a rather large size, higher than wide; spire conical, a little 

 more than equalling length of lower half of body volution. Whorls about 

 five and a half, very convex; those of the spire each showing three- fourths of 

 its entire height above the next succeeding one ; upper ones (in casts) rounded, 

 last one large and ventricose, and, like the next above, sub angular around near 

 the middle, below which it is somewhat produced, and rounds into a small, 

 umbilical opening in the cast, probably entirely closed by the columella in 

 specimens retaining the shell. Spiral band apparently of moderate breadth, 

 occupying the obtuse angle a little above the middle of body-whorl, and pass- 

 ing around near the middle of the others. Suture deep in consequence of the 

 great convexity of the volutions. 



Aperture sub- circular. Surface of internal casts showing, on the upper con- 

 vex slope of the body- whorl, and that of the next above it, obscure transverse 

 ridges, curving backwards as they extend out from the suture, probably par- 

 allel to the lines of growth. Crossing these, there is an undefined, revolving 

 ridge on the body-whorl a little more than half way out from the suture 

 towards the spiral band. Other surf ace- markings are unknown. (Copied from 

 Meek and Worthen's description in Vol. 3, of Geol. Rep. of Illinois.) 



This species is easily distinguished by the transverse striae, on the upper 

 side of its body- whorl, from all other Silurian Pleurotomaria. 



Formation and Locality. In the Niagara limestone of the quarries east of the city of Louisville, 

 Ky. A rather rare species. 



Pleurotomaria arabella. \. SP. 



Plate XXVI., figure 12. 



Shell rather large, tilfbinate, spire elevated ; apex minute, aperture sub- 

 quadrate, apparently somewhat wider than high. Volutions five or six, prom- 

 inently convex ; rapidly enlarging, last one or body-whorl very ventricose. 

 Shell wider than high. 



Surface marked by three revolving carinae, of which one is above and the 

 other two below the peripheral band ; the band itself is flat and narrow, and 

 not limited by elevated carinae ; the upper part of the volution, at least in the 

 two last ones, is gently sloping from suture to the first or upper carina; from 

 this it curves to the spiral band, forming a moderately deep, rounded furrow. 



The interspaces between band and second carina, and between this and the 

 third or last carination, are also rounded depressions, of which only the one 



