76 The Saint Peter Sandstone. 



tinguish from the concave upper surface except by noting the direction of 

 the aperture. The outer surface of each volution is gentlyconvex, and joins 

 to the upper and lower surfaces by acutely rounded angles, the lower one of 

 which is perhaps a little the stronger. The upper and lower surfaces of 

 each coil are about equally convex, and are nearly straight. The inner sur- 

 face is quite straight, is equal in length to one-fourth the outer surface, and 

 equal to once the outer surface of the volution with which it is in contact- 

 Sutures deep on the cast. Surface smooth but marked by five transverse 

 lines which run a little forward from the suture to the upper outer angle, 

 Ihence obliquely down and back to the lower outer angle and from there 

 curve a little forward to the contact with the penultimate whorl. Shell 

 probably very thin. 



Highland Park. 



Genus Platyceras. 



Platyceras vetulum n. sp. 

 Plate IV, figure 1. 



Several specimens have been found which appear to belong all to the 

 same species, although they vary in the formof theaperatureand convexity 

 of the body whorl, owing partly to distortion. In general the casts show 

 that the shells were small, dextrally coiled, and had the body whorl pro- 

 portionally very large. The spire or apex is not well preserved in any of 

 the specimens, but w^as low, scarcely rising above the body whorl, and con 

 sisted of one, possibly two whorls. The aperature was ovate, narrowest 

 on the side next the spire, and strongly indented by the penultimate whorl. 

 The plane of the aperature is nearly parallel to the axis of the spire. 



There is some resemblance in this species to very rapidly expanding 

 shells of the genus Holopea. Tin fortunately no specimens could be found 

 sufHciently well preserved, and notenoughof them,to allowof good generic 

 diagnosis. The figure is of a large specimen. 



Found at South Saint Paul. 



Genus Pleurotomaria. 



Pleutorotnaria aiens n. sp. 

 Plate III, figures 16 and 17. 



Shell large, consisting of about four volutions which are rapidly ex- 

 panding and ventricose. The first volutions on interior casts are evenly 

 rounded, but the last one on large specimens has a broadly rounded carina 

 a little below the middle. Apical angle 120°. Suture deep. Aperture large, 

 indented on the inner side by the penultimate volution. The margin curves 

 back from the suture for nearly one-fourth of the last volution and then 

 forward an equal distance, making a deep, rapidly narrowing fissure, the 

 rounded apex of which torms the longitudinal ridge. The umbilicus is large 

 and opens nearly to the apex. No surface markings except irregular 

 growth lines. ^ 



Highland Park and South Saint Paul. 



