SCO PALAEONTOLOGY OF ILLINOIS. 



rently rhombic-subquadrate in form. Surface ornamented by 

 about twenty obscure, closely arranged, revolving strias, crossed 

 by stronger, very regular transverse lines, which are most 

 distinct on the upper part of the whorls, and pass with a 

 gentle curve backward and outward to the spiral band. Below 

 the angle the under side of the body whorl is nearly smooth, 

 or only marked by very obscure lines of growth, and faint 

 traces of revolving striae. Length, about 0.93 inch; breadth, 

 nearly 0.97 inch; apical angle regular, divergence 64°. 



This shell seems to be very closely allied to P. Riddelii, of Shumard ( Trans. 

 Acad. Sci. St. Louis, vol. i, p. 625), and a more careful comparison with his 

 description leads us to suspect that it may possibly prove identical. Yet, as 

 Dr. S. describes his species as having only twelve or thirteen revolving lines 

 on each whorl, while our shell shows uniformly nearly double this number, we 

 are in doubt whether they should be considered identical or not. It is also 

 related to Pleurotomaria missouriensis—^TrocTius missouriensis, Swallow), but 

 never attains near so large a size as that noble species, from which it also 

 differs in having much stronger transverse striae ; while Prof. Swallow's species 

 has not the prominent linear ridge just above and below the spiral band, seen 

 in our shell. 



At the time we first published our description of this species, we had not 

 seen Prof. McChesney's paper in which he published it under the name P. 

 bicarinata, nor had we any knowledge of its publication until about the 10th 

 of March, 1860. Our paper was published Nov. 21th, of the same year. 

 These dates are of no consequence, however, so far as the species under con- 

 sideration is concerned, since the name under which Prof. McChesney pub- 

 lished it, had been previously used for three other species, the first as far back 

 as 1818. 



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



Pleurotomaria subscalaris, M. and W. 



PL 28, figs. 10a, 106 (by error on the plate, 9a, 96). 



Pleurotomaria subscalaris, Meek and Worthed, October, 1860. Proceed. Acad. Nat. 

 Sci., Philad., p. 460. 



Shell large, rather thick, conical-ovate ; spire moderately 

 elevated; volutions six, those near the summit of the spire 



