61 
Spirifer opimus Hall 
1858. Spirifer opimus Hall, Geol. Iowa, vol. 1, pt. 2, p. 711, PL 28, figs. 
1 a-b; Spirifera opima Hall, 1883, 2nd Rept. N.Y. State Geol. 
for 1882, PI. (31) 56, figs. 4-7; Spirifer opimus Hall and Clarke, 
1894, Int. to Study of Brach., pt. 2, PL 27, figs. 12-13; Hall and 
Clarke, 1895, Pal. N.Y., vol. 8, pt. 2, PL 31, figs. 4-7; Mather, 
1915, Bull. Sci. Lab. of Denison Univ., vol. 18, p. 185, Pl. 12, 
figs. 7-7 c. 
Remarks. There are several Minnewanka specimens representing 
this species. One pedicle valve, largely an internal mould, is 17 mm. long, 
18 mm. wide along hinge-line, and 9 mm. thick; there are three low plica- 
tions in the sinus and 8 stronger, simple ones upon each lateral slope; the 
sides are apparently almost vertical, being as wide at 5 mm. anterior to the 
cardinal extremities, as at the hinge-line. Two of these vertical-sided 
forms, a pedicle and a brachial valve, and typically opimus also in all 
other respects, have the plications adjacent to the median fold and sinus 
bifurcating ; others similar to the latter have rounded cardinal extremities, 
whereas still others have them extended. There is likewise among these 
forms a complete gradatipn from all plications of sinus much weaker than 
those upon the lateral slopes of valve, through specimens with median 
plication strong and the two or four side ones weak, to specimens with all 
strong. This is similarly true of the fold.. There thus appear to be com- 
plete gradations between the straight-sided, non-bifurcate form char- 
acteristic of the type figure of opimus to the straight and the round-sided, 
bifurcate rocky montanus. On the other hand, the typical opimus seems 
to grade into S . boonensis Swallow emend. Girty. See also remarks under 
“General Discussion of Spirifer rockymontanus” , page 57. 
Spirifer opimus Hall may represent a line descending from S. alba - 
pinensis with an accentuation of growth anteriorly and to a slightly less 
degree antero-laterally, resulting in a comparatively long, short-hinge 
form; the plications of fold and sinus enter as in centronatus, and, attain- 
ing a similar number, become as broad as the lateral plications but remain 
low, at times obscure. 
Locality and Horizon . Pennsylvanian of the Appalachian and Missis- 
sippi valley. In the Minnewanka region in the Pennsylvanian of section 
2-12 (c). 
Spirifer earner atus Morton 
1836. Spirifer cameratus Morton, Am. Jour. Sci. (1), vol. 29, p. 150, Pl. 2, 
fig. 3; Hall, 1858, Geol. Surv., Iowa, vol. 1, pt. 2, p. 709, Pl. 28, 
figs. 2 a, b; Meek, 1872, U.S. Geol. Surv., Nebraska, p. 183, PL 6, 
fig. 12, PL 8, fig. 15; White, 1875, U.S. Geol. Surv., W. 100th 
Mer., Rept., vol. 4, p. 132, Pl. 10, figs. 1 a-d; Meek, 1877, U.S. 
Geol. Surv., 40th Par., vol. 4, p. 91, PL 9, figs. 2, 2 a; White, 1884, 
Geol. Surv., Indiana, 13th Rept., p. 132, PL 35, figs. 3-5. 
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