60 
Spirifer opimus apparently occurs at various localities in Mississippi 
valley without intergradations to S. rockymontanus of either of Marcou’s 
forms. Similarly S . boonensis is developed in Colorado apparently without 
transition forms to other species. These appear to be the only forms, so 
far recorded, of the different variations of the rockymontanus type to have 
bred true at some locality. In the Minnewanka region occur connecting 
forms not only between these but between these and the following — a 
transverse form with hinge-line equalling the greatest width below and 
with bifurcating plications; an extended form like boonensis but with 
bifurcating plications; an opimus-like form with bifurcating plications; 
an alate form with much extended, rounded hinge-extremities, and plica- 
tions bifurcating; a transverse form with rounded hinge-extremities and 
simple plications. These limiting forms may be found to have bred true 
in some other region, but until these places are discovered, they are treated 
merely as variations of S. rockymontanus. 
This brings us back to the question of the expediency of separating 
Marcou’s type figures into two species. Aside from the two forms repre- 
sented by opimus with the plications of fold and sinus obscure and boon- 
ensis with its extended hinge-line, and both with simple plications, there 
are apparently transitions between the other forms in all regions so far 
known, that is, between those with simple plications and rounded hinge- 
extremities and all forms with bifurcating plications. These intergrading 
types vary greatly also in size. Since thus neither of the two forms repre- 
sented by the two specimens figured by Marcou (the smaller one with 
rounded cardinal extremities, plications simple, those of sinus and fold as 
strong as the lateral ones, the larger form with hinge-line apparently as 
long as the shell below, with bifurcating plications), occurs distinct in any 
area, without specimens transitional to the other, we must consider them 
as representing a single species. 
The specimens Girty has figured from Colorado 1 approach rather close- 
ly the smaller form, but have bifurcating plications; this is similarly true 
of S. rockymontanus figured by Raymond from Pennsylvania 2 ; Mather’s 
specimens from Arkansas and Oklahoma have bifurcating plications, and 
approach Marcou’s larger form, but the hinge-line varies from less to greater 
than the width of shell below, that is, from rounded to extended cardinal 
extremities. 
Spirifer boonensis Swallow emend Girty 
1860. Spirifer boonensis Swallow, Acad. Sci., St. Louis, Trans., vol. 1, p. 
646; Girty, 1903, U.S. Geol. Surv., Prof. Paper 16, p. 381, PI. 6, 
figs. 1, 2, 3; Price, 1914, W. Va. Geol. Surv., Kanawha co., Rept. 
p. 652, PI. 2, figs. 1-3. 
Remarks. The Minnewanka specimens are apparently identical with 
the form described and figured by Girty from Colorado, though our speci- 
mens average 10 to 11 lateral plications, whereas those from Colorado 
average 12 to 13. There seem to be perfect gradations between S. boon- 
ensis and S . rockymontanus , form C, described above. See also “General 
Discussion of Spirifer rockymontanus ,” page 57. 
Locality and Horizon. Pennsylvanian of West Virginia ?, Missouri, 
Colorado. In the Minnewanka region in the Pennsylvanian of section 2-12 (a) . 
1 Loc. cit.,1903. 
a Loo. cit., 1910. 
