S«9 PALJEONTOLOGY OF NEW-YORK. 



Plat}ostoiiia aronosa. 



Plate LVII. Fig. 3 a, 6. 



Platyostotna arenosa : Conhad, Jour. Acad. Nat. Sciences of Philadelphia, 1842, Vol.viii, pa. 276, 



pi. 17, f. G. 



" Obliquely ovate-globose : body-whorl rapidly widening to the base ; 

 " aperture longitudinally suboval." 



The above description of Mr. Conrad corresponds with the characteristics of this 

 and other specimens in my collections, which, however, appear to me as the young 

 of P. ventricosa. As the specimens are mostly casts, or only preserving the shell in 

 a very imperfect condition, it is impossible to determine this question at tlie present 

 time with entire satisfaction. 



Fig. 3 a. View looking upon the spire. Fig. 3 b. View of the aperture of the same. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Near Catskill. 



There are, besides the true PLATTOSTOMiE, some other shells of not very 

 dissimilar character in the rocks of this age. Some of these, in the casts 

 and in their exterior conformation, are remarkable for their oblique form 

 and the wide-spreading of the last volution, which is also often exagge- 

 rated by pressure in the same direction. 



These fossils have sometimes the globose form of Platyostoma ; but in 

 these instances they are usually more symmetrical, and may be recognized 

 by the smoothly rounded outline and the extreme posterior extension of 

 the outer lip on the adjacent volution. The surface is evenly striated by 

 fine elevated threadlike striae parallel to the lines of growth. 



Although differing in external characters from any other gasteropods 

 of the group, I have not until recently had the means of determining the 

 generic relations of these forms. During the past year (1857) I obtained 

 from Mr. Andrews of Cumberland, Maryland, some Gasteropoda of the 

 Oriskany sandstone, among which was a single specimen of one of these 

 shells entire and without adhering stone. This specimen, and some others 



