584 Contributions to Invertebrate Palseontology. 



given. They also differ much in the size and strength of the mesial 

 elevation and sinus, and in the length of the hinge-line. I have 

 much doubt as to the absolute identity of this shell with the Illinois 

 shells described and figured by Messrs. Meek and Worthen, from 

 the fact that these appear distinctly marked by the radiating striae, 

 while those from the west are said to be smooth, except for the 

 concentric lines of growth, though occasionally show^ing faint evi- 

 dences of obscure radiating lines. The fact that the authors of that 

 species refer it to Sp. glaber, which is entirely destitute of radiating 

 lines, would seem to indicate it as different from the Ohio forms. 



Spirifera Rockj'iuoutana ? 



Plate XIII, fig. 20. 



Spirifera Rockymontana Marcou, Geol. N. Aiuer., p. 50, pi. 7, fig. 4; Feb. 1858. 

 Spirifera Keokuk Hall, Geol. Kept. Iowa, vol. 1, pt. 2, p. 642, pi. 20, fig. 3; 



Sept. 1858. 

 Spirifera Keokuk var. Hall, Ibid., p. 672, pi. 24, fig. 4. 

 Spirifera Opima Hall, Ibid., p. 711, pi. 28, fig. 1. 



Several specimens of a Spirifera, of the form referred to S. Keo- 

 kuk var. by Prof. Hall, have been obtained from Newtonville, Ohio, 

 which are so entirely similar to those from the St. Louis and Chester 

 limestones of Iowa, as to be absolutely undistinguishable; the form 

 of the shell, the form and number of the plications, and the minute 

 surface structure being exactly as in those. 



The form of the shell will vary from longer than wide to much 

 wider than long, dependent on the extension of the hinge-line. In 

 profile the shell is extremely ventricose, with a strongly enrolled 

 beak ; a moderate cardinal area, vertically striated ; a well-marked 

 mesial fold and sinus; from seven to ten simple, rounded, or sub- 

 angular plications on each side, and from four to six bifurcating or 

 dividing plications on the fold and sinus. The plications and inter- 

 vening spaces, when the surface is well preserved, are marked by 

 fine longitudinal lines, showing even on partially exfoliated speci- 

 mens, and are also crossed by still finer transverse striae which un- 

 dulate in crossing the plications, and on perfectly preserved surfaces 

 appear to be minutely setose on their edges. 



The species is extremely variable in its general outline, as ex- 

 hibited among the collections from all of the many localities from 

 which I have examined specimens, especially in the extension of the 

 hinge-line, and the proportional width of the shell below, and also 



I 



