57 
Form C 
Shell transverse, hinge-line extended forming the greatest width of 
the shell. From the front of the shell the lateral margins extend either 
straight or more usually with a gentle convexity to the cardinal extremities. 
Plications rather angular, 1 or 2 adjacent to fold and sinus bifurcating. 
This form is in outline, size, and angularity of plications, apparently 
identical with S. boonensis, but it has bifurcating plications, which the 
latter has not. The proportion of length to breadth of each is about 
5 to 8 or 9. Represented by g and h in the table, page 54. 
Form D 
Shell transverse, width much greater than length, often about 2 to 1; 
cardinal extremities extended (i and j in the table, page 54) or rounded (k 
in table). Plications usually subangular, the pair adjacent to the median 
sinus and fold bifurcating. 
Form E 
Shell transverse, width greater than length, in the proportion of 
about 7 to 5. Cardinal extremities rounded. Plications rounded, those 
upon the lateral slopes simple. This appears to be identical with White's 
figures 9 a-c 1 , agreeing with it in all observable respects. 
No form with cardinal extremities mucronate was noted. 
General Discussion of Spirifer rockymontanus 
From a study of Marcou's illustrations it is evident that his figures 
4-4 b and 4 c-e refer to different forms, the former a rather large shell 
with a hinge-line apparently equalling the greatest width of the shell 
below and with bifurcating ribs, the latter smaller with the hinge-line 
distinctly shorter than the shell below, and with simple ribs. It is to the 
latter type that his description conforms the more closely, as may be seen 
from his original description fully given below. 2 “Shell transverse, gibbous, 
subsemicircular, length and breadth nearly equal, cardinal angles rounded. 
Beak large, elevated, incurved; hinge-line shorter than the width of the shell, 
straight. Sinus of the dorsal valve shallow, extending from the beak 
to the margin, covered with ribs, like the other parts of the two valves. 
The ribs are simple , sharp, and very distinct. Area small and triangular. 
The lines of growth are indistinct on the two valves." The italics are 
ours. The plications of the smaller shell are the more angular; those of 
sinus and fold of both forms are practically as strong as the lateral ones. 
The figure of the smaller specimen has an area extending the length of 
the hinge-line; the larger one probably had a similar extent, but gives 
evidence here of imperfect preservation and the area is depicted as being 
triangular and short; it was probably similar to the other. It is, conse- 
quently, for the latter form (4 c-e) that his name should be retained if 
it is found expedient to separate them. This is similarly the opinion of 
1 Loo. cit. 
1 Loo. cit. 
