ORISKANY SANDSTONE. '4©9 



incipient tortuous columella, one or two specimens approach the Genua 

 Strophostylus. 



Although the discrimination of species is attended with some difficulty 

 when few specimens are examined, there has nevertheless appeared, in 

 the large collections obtained, sufficient reason to regard the species 

 indicated as well established. 



Platyostoma Tentricosa. 



Plate CXII. Fio. 1-10; Plate CXIII. Fig. 7 & 8 ; and Plate CXV. Fig. 8. 



Platyostoma ventncosa : Con'rad, Jour. Acad. Nat. Sci. Philadelphia, Vol. viii, pa. 275, pi. 17, f. 1. 

 — — This volume, pn. 300, pi. 55. 



Shell globose or depressed-globose and often obliquely ovoid, varying in 

 form. Spire moderately elevated, consisting of three or four volutions, 

 the last of which is extremely ventricose : volutions flattened upon 

 the upper side ; aperture circular or subovate ; columellar lip reflexed. 



SuBFACS marked by fine closely arranged striae parallel to the lines of 



growth. 



The surface is sometimes marked by broad undulations, both in young and In old 

 shells; but I am not able to find in these forms any means of specific distinction. 



PLATE CXII. 

 Fig. 1 & 2. Young shells of this species. 



Fig. 3. A young speoiraen, showing a broad undulating surface. 

 Fig. 4 a. View of the aperture of the same. 

 Fig. 4 6. An older sfccimcn of the ordinary form. 



Fig. 5 a, h. View of the spire, and of the aperture of a specimen of medium size. 

 Fig. 6 c. Profile of a specimen which is somewhat depressed vertically. 

 Fig. 6. A young specimen with a more elevated spire. This is of the type of P. artnosa of 

 CoNBAD (See page 302, this volume). 



Fig. 7 & 8. View of the spires of two specimens of ordinary form. 



Fig. 9. A specimen which has been vertically compressed. 



Fig. 10 a. h. View of a specimen which is a little compressed obliquely. 



PLATE CXIII. 



Fig. 7. Profile of the spire of a very large specimen, which has been somewhat compressed 

 vertically. 



Fig. 8. View of the spire of the same specimen. The surface shows broad undulations pa- 

 rallel to the lines of growth. 



