20 [Assembly 



OBSERVATIONS 



ON THE GENERA 



PLATYOSTOMA AND STROP H OSTYLUS. 



PLATYOSTOMA. 



The Germs Platyostoma was described by Mr. Conrad in the 

 Journal of the Academy of Natural Sciences of Philadelphia in 1842, 

 Vol. VIII, p. 275, as follows : 



'< Platyostoma ( Conrad ) 



" Shell subglobose : spire short ; aperture very large, suborbicular, 

 " dilated ; labrum joining the body whorl at right angles to the axis 

 "of the shell." 



The Platyostomse are globose shells with low spires, having colu- 

 mella : the last volution is extremely expanded, the aperture very large, 

 and the columellar lip thickened. 



These shells are often distorted by pressure to a great extent, and it 

 is not always possible to distinguish the casts of this genus from those 

 of the Strophostylus, or from those of some species of the Platy- 

 CERAS of Conrad. 



The P. ventricosa is given as the tvpe of the genus, and the accompanying 

 figure is copied from the outline of the shell as given by Mr. CoNKAD on 

 Plate XVII of the volume cited. 



Platyostoma ventricosa. 



Under this genus Mr. Costrad described three species, the Platyostoma 

 ventricosa and P. arenosa from the Oriskauy sandstone, and the P. lineata 

 from the Hamilton group. 



