1890.] NATURAL SCIENCES OF PHILADELPHIA. 169 



Platyceras (juincyense McChesney, 1867. Trans. Chicago Acad. Sci., vol. I, p. 

 49, pi. vi, figs. 6a, 6b. 



Platyceras [Ortkonychia) cjuincyense Meek ^; Worthen, 1868. Geol. Sur. 

 Illinois, vol. Ill, p. 510, pi. XV, figs. 5a, 5b. 



Platyceras quincvense Keyes, 1889. Proc. Acad. Nat. Sci., Phila., 1889, p. 

 290. 



Capiilus quincyoisis Keyes, 1890. Am. Geol., vol. V. 



Shell of medium size, broadly conical, often more or le.ss elon- 

 gated ; expanding very rapidly and regularly from the central or 

 subcentral apex to the aperture. Usually five broad, rounded ridges 

 extend from near the apex to the aperture, which is consequently 

 Biore or less prominently quinquelobate ; the ridges are not unfre- 

 quently further divided into two or more smaller folds. Lip sharp, 

 sinuous. Surface marked by subimbricating lines of growth and also 

 by numerous small, often undefined, longitudinal co.sta3 which do not 

 appear in the cast. 



Horizon and localities. Burlington limestone : Burlington, Iowa ; 

 and Quincy, Illinois. 



The specimens described by ]\rcChesney and by Meek and Wor- 

 then were either exfoliated examples or internal casts ; and this is 

 the condition in which the species is usually found. Owing to the 

 peculiar state of pre.servation the shells quickly crumble away in 

 handling, leaving only the internal casts, but the distinctive quin- 

 quelobate character always renders them easily recognizable. In 

 the examples figured l)y McChesney and also by Meek and "Worthen 

 the apices were wanting, but the individuals were not as imperfect 

 as was supposed. 



During the earlier periods of their growth many of the shells of C. 

 quincyensis were very broad, but when attaining about one-third 

 their maximum size the aperture abruptly became relatively smaller, 

 leaving a sharp subangular ridge around the shell parallel to the 

 apertural margin. This abrupt decrease in the expansion of the 

 shell impax'ts to the natural internal casts the appearance of an apical 

 truncation or fracture. 



In its attachment to paleozoic crinoids the onl}' forms with which 

 C. quincyensis has thus far been found associated is Physetocvinus 

 ventricosus (Hall), a species having a rather depressed hemispherical 

 dome, in which the ventral opening has a subcentral location. The 

 dome plates are small and numerous and frequently studded with 

 small prominent tubercles or subspinous processes, which impart to 

 the gasteropod shell series of minute corrugations extending over 

 each of the larger folds. 



