GASTEROPODS. 191 



Orthonychia chesterense (MEEK <fe WORTHEN). 

 Plate iiv, figs. la-d. 



Platyceras (Orthonychia) chesterense Meek & Worthen, 1866: Proc. Acad. 



Nat. Sci., Phila., p. 265. 

 Platyceras chesterense Keyes, 1888: Proc. Am. Philosophical Soc., vol. 



XXV, figs. 4-5. 



Capulus chesterensi* Keyes, 1890: American Geologist, vol. VI, p. 9. 

 Capulus chesterensis Keyes, 1890: Proc. Acad. Nat. Sci., Phila., p. 176, 



pi. ii, figs. 13a-d. 



Shell quite small, obliquely conical, with usually five more 

 or less well-defined longitudinal furrows, which alternate with 

 broad, flattened folds. 



Horizon and localities. Lower Carboniferous, Kaskaskia 

 limestone : Ste. Genevieve. 



This little species is found almost invariably attached to 

 the vaults of crinoids ; and Meek & Worthen report one adher- 

 ing to the side of Pentremites godoni DeFrance, " so as to en- 

 tirely cover one of the pseudo-ambulacral fields and two 

 intermediate areas." The specimens from Kentucky are nearly 

 all attached to Pterotoerinus P. acutus, P. bifurcatus and P. 

 depressus. In the first of these species the ventral side is 

 very much elevated, being nearly three times the height of the 

 dorsal cup. The first radial dome-plates are produced into 

 monstrous alate processes, leaving only a small summit, which 

 is perforated for the anus. The margin of the gasteropod 

 shell has followed closely the surface in contact; and in the 

 majority of cases, when the apical part of the crinoidal vault 

 was not sufficiently extensive for the support of the enlarging 

 shell, the apertural margin has been prolonged into the inter- 

 radial depressions, forming prominent, rounded, linguiform 

 extensions ; while the protruding upper edges of the alate 

 dome-plates of the crinoid have given rise to five deep, 

 broadly rounded sinuses in the lip of the molluscan shell. The 

 lines of growth in the shell are therefore extremely sinuous, 

 the undulations in the direction of the aperture being concave 

 on the broad, flattened folds, and convex in the furrows. In 

 some specimens the furrows and folds have their origin near 

 the apex a fact which is suggestive that the forms of the shell 



