58 
Girty, 1 but Mather 2 would restrict it to the form represented by figures 
4-4 b, because these are the first figures cited upon the plate. Common 
usage has inclined to the former opinion but has also included individuals 
under rockymontanus , which show two or three plications upon a valve 
adjacent to the median fold and sinus, bifurcating once from the beak 
forward, and also forms with a slightly extended hinge-line. Thus White 3 
figures two specimens, the one, 9 a-c, has the plications somewhat rounded 
and non-bifurcating, whereas 9 d has more angular plications, two or three 
of which, adjacent to the fold, bifurcate; the former has the hinge-line 
equalling the greatest width of the shell below, the latter slightly less 
than the shell below, and with more angular plications. 
The form Girty 4 figures from Colorado has the hinge distinctly shorter 
than the shell below, and with a few lateral plications bifurcating. Two 
other forms that enter into this discussion are S. opimus Hall, usually 
considered a synonym of S. rockymontanus , and S. boonensis Swallow as 
interpreted by Girty 6 . The former, as shown by Hall's figures, has length 
and breadth equal, with hinge-line equal to the width below, 23 mm., 
sides of shell almost vertical, lateral plications simple, angular, those 
upon fold and sinus weaker than the rest. S. boonensis has hinge-line 
distinctly longer than width of shell below; a large shell measures 40 mm., 
antero-lateral angles oblique, lateral plications simple, those of fold and 
sinus somewhat weaker than the others. Mather 6 has revived S. opimus 
for some specimens found in association with S. rockymontanus; his figures 
approximate Hall’s strength of plications, but have a more oblique antero- 
lateral angle, approaching in this respect boonensis. Some of our specimens 
approach in outline and size very closely the small rounded form figured 
by Marcou in 4 c-e as rockymontanus, but these pass by imperceptible 
gradations into the almost vertical sided type figured by Hall 7 , as opimus, 
thence into the more transverse form shown by White in figure 9 d 8 , or Mar- 
cou’s rockymontanus figures 4 a-c, from this into the type shown in White’s 
figures 9 a-c, thence into Girty’s boonensis ?, figure 3 9 and from this into 
Girty’s form boonensis as shown in figures 1 and 2. The fold and sinus 
vary in strength and in their plications from rounded to angular; the 
lateral plications also vary in angularity, similarly in the number bifur- 
cating, from none to 3 or 4. There is thus an apparent gradation in 
all essential features between the various forms noted above. These 
variations were noted upon specimens taken from the prolific brachiopod 
bed one foot thick of section 2-12. White 10 evidently found in Utah and 
New Mexico similar variations, for he says “The numerous specimens 
of this species in the collections present most perplexing variations, and 
I have repeatedly been almost persuaded to arrange some of them under 
a separate specific name, but the presence of intermediate forms has 
prevented such a decision Many of the examples under 
examination agree in all essential particulars with S. opimus Hall, which 
thus becomes a synonym of S, rockymontanus.” 
1 Loc.cit,, 1903. 
•Loc. cit., 1915. 
* Loc.cit., 1877. 
* Loc. cit., 1903. 
* Loc.cit. ,p. 381. 
* Loc. cit., p. 185. 
7 Geol. Iowa, vol. 1, pt. 2, p. 711, PI. 28, figs, la, b. 
•Loc. cit., 1875. 
•Loc. cit., 1903. 
lC Loc. cit., p. 135. 
