60 



PlatyostOxMA affinis. (N. sp.) I 



Plate 5, fig. 2. 

 (Compare P. ventricosa, Conrad.) 



Description- — Shell depressed, conical ; of about four rounded whorls, 

 the last of which sometimes exhibits a tendency to become vagrant ; apical 

 angle about 90^. Surface with fine transverse stride, and often with a 

 few obscure undulations. Width of a large specimen at the base, two,and 

 one-half inches ; height about the same. 



The specimens are all casts of the interior, with only faint indications 

 of the surface markings. In some the whorls are evenly rounded, the 

 most projecting part being about the middle. In others the most promi- 

 nent point is below the mid-height of the whorl, and in such the spire is 

 somewhat oblique. In large specimens, the aperture is expanded down- 

 wards, as shown in fig. 2, pi. 5. The umbilicus has not been distinctly seen, 

 but from appearances it must be very small. When a considerable por- 

 tion of the last whorl is broken off, the shell appears to be more erect as 

 in the followinor fissure- 



Fig. 30. 



Such specimens when compared with a large individual, might be taken 

 for a distinct species. Often the spire is somewhat more depressed and 

 rounded. The body whorl is much larger in some specimens than it is 

 in others. 



This species is closely allied to P. ventricosa of the Oriskany sandstone. 

 It differs therefrom in]having a more elevated spire and in being more 

 erect. Some specimens are so much like P. ventricosa, that it would be 

 difiicult to point out sufficient to separate them. But when we place a 

 group of a dozen specimens^ of P. ventricosa alongside of a similar 



