52 
Acceleration in the Recapitulation of the Albapinensis Stage in Spirifer 
Centronatus , from Successively Later and Later Beds 
Section 
number 
Average distance from beak in 
millimetres for insertion of 
the first plication 
Lateral 
Median 
S. centronatus 
23 
4 
3 
25 
5 
4 
29 
8 
7 
32 
8 
7 
34 
9± 
7± 
S. albapinensis 
35 
Normally neither lateral nor 
median plication present; at 
times one or both appear at the 
anterior margin of a mature 
individual. 
There was here apparently a gradual change of the main mass of 
the albapinensis line into the centronatus form. A few retarded specimens 
of the former continue to occur even after the latter has become long 
established as the dominant group, like paleolithic humanity in the midst 
of our modern civilization. These disappear before centronatus reaches 
its maximum development and, apparently, without descendants. 
S. centronatus minnewankensis may possibly be due to the de Vries 
mutation factor, as no transitional forms were noted. 
Locality and Horizon. Lower Mississippian of Ohio, Colorado, Utah, 
Nevada, Wyoming, and Montana. In the Minnewanka region in the 
Lower Mississippian of sections 2-22 (c), 23 to 25 (C), 29 (c), 30 (c), 32 (c), 
33 (r), 34 (r); 4-2 (r). 
Spirifer centronatus minnewankensis n. var. 
Plate I, figure 8a, b 
" Description. An alate but non-mucronate variety of S. centronatus. 
Hingerline forming the greatest width of shell. Length of type (a pedicle 
valve) 18 mm., width along hinge-line 40 mm., convexity 7 mm., with 18 
plications upon each side of median sinus, and 3 in the sinus. An entire 
shell from the same locality measures similarly 20 mm. by 45 mm. by 9 
mm., the thickness of the combined valves being 15 mm. ; there are appar- 
ently 18 plications upon each side of the sinus with 3 in the sinus, except at 
the very anterior edge where there are 5 ; median fold with 4 plications. 
Pedicle valve most strongly arched at umbo, incurvature as in S. 
centronatus , but lateral slopes to cardinal angles much less abrupt, forming 
an almost straight line. Area comparatively narrow, arched. Plications 
low, 16 to 20 upon each side of sinus. Median sinus shallow, rather narrow, 
with 3 to 6 plications less conspicuous than those upon the lateral slopes. 
Brachial valve much less arched than the pedicle, lateral slopes and 
plications similar. Median fold low, rather narrow, with 4 to 6 plications, 
less prominent than those upon the lateral slopes. 
The finer surface markings consist of concentric growth lamellae and 
fine radial lines. 
