PALAEONTOLOGY OF ILLINOIS. 



from Prof. McCHESNEY's description, until we hart an opportunity to 

 compare it with good examples of that species from the original locality. 

 On comparison with these, we find onr shell to be readily distinguished 

 by having its spiral band located midway between the middle and 

 upper margin of the body whorl, instead of passing around the middle 

 of the outer side. It likewise differs in showing no traces of revolving 

 strife on the spiral band, and in having small wrinkles across the revolv- 

 ing strife on the under side of the body whorls, while the little wrinkles 

 around the upper edge of the whorls are stronger and shorter than in 

 P. Becktritliana. Again, there is a difference in the revolving stria- 

 those of onr shell never having an intermediate, smaller one between 

 two larger ones, as is generally the case with those of Prof. McCHES- 

 NEY'S species. 



The close similarity between these two species, both in form and orna- 

 mentation, shows the necessity for great care and precision in drawing 

 up descriptions of specimens, even where they may be widely different 

 from all known forms, since we often find in such cases that other really 

 distinct species are afterwards discovered that cannot be distinguished 

 by the original description from forms already described. 



Locality <ai<l position Lower member of the Coal Measures on Hodge's 

 creek, Maconpin county, Illinois. 



PLETJROTOMAKIA^ VALVATIFORMIS, M. and W. 



PI. 29, Fig. 9. 

 Plevrotomaria vahatiformis, MEEK and WORTHEN, 1866. Proceed. Acad. Nat. Sci., Phila., p. 273. 



SHELL minute, depressed, or about twice as wide as high ; 

 volutions three and a half to four, regularly rounded, and 

 increasing rather gradually in size ; suture well defined, in 

 consequence of the convexity of the whorls ; umbilicus pro- 

 portionally small, or closed; aperture sub-orbicular, being 

 a little straighter on the inner side. Spiral band nearly or 

 quite even with the surface of the whorls, and placed on 

 the middle of the outer side. Surface smooth, as seen with- 

 out a magnifer, but presenting traces of microscopic revolv- 

 ing stride, in a good light under a strong lens. 



Hight, 0.04 inch; breadth, 0.08 inch. 



This is by far the smallest Plcurohnimrio (if it really belongs to that 

 genus) we have ever seen, and if it were not for the fact that we find 



