370 PALAEONTOLOGY OF ILLINOIS. 



Macrocheilus medialis, M. and W. 



PI. 31, fig. 5 a, 5 b. 



Macrocheilus medialis, Meek and Worthen, October, 1865. Proceed. Acad. Nat. Sci., 

 Philad., p. 466. 



Shell of medium size, rather thick, rhombic-subovate ; spire 

 depressed conical, acutely pointed at the immediate apex, form- 

 ing about one-third the entire length ; whorls six to six and a 

 half, convex, increasing rapidly in size, last one large, but not 

 very ventricose, the widest part being near the middle; suture 

 moderately distinct ; aperture rather narrow subovate, or sub- 

 rhombic, acutely angular above, and narrowly round or slightly 

 effuse at the base of the columella below; outer lip sharp, 

 nearly straight in outline ; columella somewhat tortuous, and 

 more or less sinuous about half way up the aperture; inner lip 

 thickened to the top of the aperture, and showing a slight ten- 

 dency to form an obtuse fold or prominence just below the 

 sinuosity at the middle of the columella, marked by regular, 

 obscure, transverse wrinkles or strise above the middle. Sur- 

 face apparently smooth, but showing under a lens obscure traces 

 of fine lines of growth. Length, 1.17 inches ; breadth, 0.83 

 inch; length of aperture, 0.72 inch; breadth of ditto, 0.37 

 inch ; apical angle convex, divergence 83°. 



On comparing what we believe to be authentic examples of Macrocheiles 

 primigenius, Conrad (sp.), sent by Dr. R. P. Stevens, from the Coal Measures 

 near Pittsburgh, Pennsylvania, with the form under consideration, we find that 

 they differ from our shell in having the body whorl more ventricose, the revolv- 

 ing fold on the columella more distinct, and the broad spiral furrow above it 

 deeper, and continued down around the base of the body whorl. These Penn- 

 sylvania specimens, also show no traces of the small transverse wrinkles, or 

 striae, seen on the callous of the inner' lip of our species above the middle of 

 the aperture. 



The engraver did not follow carefully enough the shading of the drawing, in 

 fig. 5 a, of our plate 31, representing the aperture of this shell. This defect 

 makes the columella look too flat and straight in the figure. 



Locality and position : Upper Coal Measures; Springfield, Illinois. 



