42 Trans. Acad, Sci. of St. Louis. 



way between the peripheral band and the suture, there is an 

 angular, revolving carina, and on both sides of it, between it 

 and the peripheral band on the one side and the suture on the 

 other, the shell is concave. A short distance below the peri- 

 pheral band there is a prominent, flattened, revolving band. 

 Between this band and the periphery the shell is concave, but 

 below it is convex, rounding over gradually into the umbilicus. 

 The number of whorls cannot be determined as only a portion 

 of the outer one is preserved. 



This shell does not properly belong to the genus Plenroto- 

 maria, but it is one of the Pleurotomaridae and is placed here 

 provisionally until more perfect specimens can be had. 



Platyschisma missouriensis n. sp. 



PI. v.f. 1-4, 



Shell composed of six whorls, umbilicate, with a low spire, 

 the diameter 44 mm., and the total height 30 mm. Each 

 whorl of the spire is about, or a little more than one-half cov- 

 ered by the next outer one, the exposed portion being flattened 

 above and rounded laterally down to the well marked suture. 

 The outer whorl is somewhat depressed so that the cross 

 section is subelliptical in outline, but towards the aperture it 

 is slightly expanded, the aperture itself being subcircular in 

 outline, with a height of 22 mm. and a width of about 23 

 mm. The margin of the aperture is broadly sinuate near the 

 columella, it then projects forward with a rounded outline to 

 a point a little below the middle of the outside of the whorl 

 where it again becomes slightly sinuate. Above this it again 

 projects slightly forward for a short distance and then curves 

 backward somewhat abruptly to form the rather narrow and 

 deep sinus which is situated at the upper part of the outer 

 side of the whorl, from this sinus the margin continues in a 

 nearly regular convex curve to the point of junction with the 

 second whorl. If straightened out the margin of the aperture 

 would have the form of the accompanying curve (pi. V, 

 f» 4), the upper end of the line being at the suture between 

 the outer whorl and the next inner one, and the lower being 

 at the columella. 



The surface of the specimen is for the most part smooth 



