51 
Remarks. The type specimen from the Keokuk of Illinois has, 
according to Weller, a length of 19*5 mm. and a breadth of 21*8 mm. 
The Minnewanka form is somewhat smaller and narrower; it also has 
numerous, minute radiating striae which are not mentioned in either 
Hall's original description or in Weller's re-description of a sulphur cast 
of the type specimen. Such minute surface markings are, however, 
easily destroyed and the difference in size and proportion are only slight. 
Locality and Horizon. In the Lower Mississippian of Illinois. In the 
Minnewanka region in the Lower Mississippian of sections 2-34 (r), 35 (c); 
4-3 (r). 
Spirifer centronatus Winchell 
Plate II, figures 1-14 
1865. Spirifer a centronata Winchell, Acad. Nat. Sci. Phil., Proc., p. 118; 
Spirifer centronatus White, 1875, U.S. Geol. Surv., W. 100th Mer., 
Rept,, vol. 4, p. 86, PI. 5, figs. 8 a-c; Spirifera centronata Hall and 
Whitfield, 1877, U.S. Geol. Expl. 40th Par., Rept., vol. 4, p. 254, 
PI. 4, figs. 5, 6; Spirifer centronatus Girty, 1899, U.S. Geol. Surv., 
Mon. 32, pt. 2, p. 547, PI. 70, figs. 3 a-d. 
Remarks . The Minnewanka specimens agree fully with this species 
as developed in Montana and also apparently with those described and 
figured by Girty from Yellowstone National park, by White from Nevada, 
and by Hall and Whitfield from Utah. 
S. increbescens from the Chester of Mississippi valley has plications 
on fold and in sinus less distinct, of more indefinite outline, the lateral 
plications broader, those bounding the medial sinus not so prominent as 
in centronatus. In S. keokuk from the Keokuk of the same region the 
shell is proportionately longer. 
Spirifer centronatus is a direct descendant of S. albapinensis; the only 
differences between them lies in the usually smaller size of the latter, 
in the character of the sinus and fold, and in the plications immediately 
bounding these. Spirifer albapinensis has the sinus smooth, or with a 
median plication and two weak lateral ones, whereas the plications bounding 
the sinus are much broader than any others upon the shell; the fold simi- 
larly is composed of usually only two plications of equal size. This form 
occurs abundantly in limestone beds 200 feet thick to the exclusion 
of S. centronatus, and the latter first makes its appearance in abundance 
after the deposition of some 380 additional feet of limestone; it then con- 
tinues as the most abundant spirifer and to the entire exclusion of S. alba- 
pinensis during the deposition of 470 more feet of limestone. In these 
beds, especially the lower ones, the small, young shell is still a typical 
albapinensis , only in later gfototh, represented in the mature shell from 
the anterior part of the umbo forward, do these bounding plications become 
less strong, and the plications in sinus and on fold increase in number and 
strength, producing the typical centronatus form. This retention of the 
albapinensis stage for a shorter and shorter time is illustrated in the 
following table and in Plate II. 
