1889.] NATURAL SCIENCES OF PHILADELPHIA. 289 



Less depressed or flattened dome. Like all the individuals resting 

 on Hat crinoidal vaults, the shells of this species, when thus situated, 

 are very much more depressed than in the normal specimens. The 

 following summary of the habits of certain Platycerata may he here 

 restated: (1) many species of Platyceras were stationary during 

 life ; (2) the nourishment of these gasteropods was derived in part, 

 at least, from the excrementitious matter of the echinoderms to 

 which they were attached ; (3) the surface of attachment governs in 

 great measure the form of the shell and the shape of its aperture. 

 From the evidence at hand it is probable that the genus in America 

 did not survive beyond the close of the Paleozoic. 



Platyceras fissurella Hall. 



Platyceras fissurella Hall, 1859. Geol. Iowa, vol. I, Supp., p. 

 90. 



Platyceras fissurella Meek and Worthen, 1873. Geol. Sur. Illinois, 

 vol. V, p. 529. 



The srecies under consideration was first obtained from the 

 Keokuk rocks at Nauvoo, Illinois ; but it appears also to be rep- 

 resented in the upper Burlington. As shown conclusively by Meek 

 and Worthen the apical portion is not perforated naturally, as 

 supposed by Hall ; the opening at the apex being merely an 

 accidental fracture. 



Platyceras infundibulum Meek and Worthen. 



Platyceras sub red ion Hall, 1859. Geol. Iowa, vol. I, Supp., p. 89 

 I pre-occupied I. 



Platyceras infundibulum Meek and Worthen, 1866. Proc. Acad. 

 Nat. Sci., Phil.', p. 266. 



Platyceras infundibulum Keyes, 1888. Proc. Am. Philosophical 

 Soc, vol. XXV, p. 238. 



The form for which Hall proposed the name P.subrectum, in the 



supplement to the Iowa report, was altered by Meek and Worthen 



to P. infundibulum, the first term having been preoccupied. Like 



P. equilaterum this species is widely distributed over the interior 



basin, chiefly through the Keokuk rocks. In the upper Burlington 



limestone it has been found adhering to the anal side of Eucladocrinus 



millebrachiatus W. and Sp. The blue Keokuk shales of Crawfords- 



ville, -Indiana, have afforded for study an extensive series of this 



species ; and the effect of its station in changing the shape of the 



shell and in giving rise to the great diversity of forms observed, has 



been very satisfactorily made out. 



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