62 
Remarks. Our specimens are normal in size and ornamentation. 
Some specimens from Colorado and Kansas are less fasciculate than ours 
and many are more so. Many specimens have a more extended hinge-line, 
some even have mucronate extensions to the hinge-extremities. S. camer- 
atus was very probably evolved from S. rockymontanus or some closely 
allied form. This is indicated by its outline, convexity, number of lateral 
plications and their elevation in the umbonal region. It is also indicated 
by the inception of plications within the sinus and upon the fold and by 
the striated to pustulose surface. The narrow, early, umbonal area of S. 
cameratus would thus represent the accelerated adult state of S. rocky- 
montanus; in these the plications of fold and sinus are similar and those of 
the lateral slopes are alike in number and elevation, with one or two 
adjacent to the sinus and fold bifurcating; it is only in the middle or an- 
terior part of the umbonal region that trifurcation with accompanying 
fasciculation takes place, and it is only very high in the Pennsylvanian, 
where much repetition has caused acceleration, that fasciculation occurs at 
the umbo. 
Locality and Horizon . A common form in the Pennsylvanian of 
Mississippi valley and the Rocky mountains. In the Minnewanka region it 
occurs in the Pennsylvanian of sections 1-29 (r), 31 (r); 2 a-3 (c), 10 (c), 
14 (c); 2-2 (r), 17 (r). 
Genus, Cyrtia Dalman 
Cyrtia standlyensis n. sp. 
Plate I, figures 2 a, b, c, d, e; 3; 4 a, b; 5 
1913. Cyrtia cyrtiniformis Shimer, Bull. Geol. Soc. Am., vol. 24, p. 236. 
Transition beds from Lower Banff shale to the Lower Banff lime- 
stone; Minnewanka region, Alberta. 
Description . Shell of medium size, subpyramidal with greatest con- 
vexity at the umbo. Hinge-line equalling the greatest width, cardinal 
extremities angular. The dimensions of a shell of average size (the type) 
are: length from hinge-line to frontal margin 20 mm., from beak of pedicle 
valve to frontal margin 23 mm., from beak of brachial valve to frontal 
margin 21 mm., greatest width (at hinge-line) 21 mm., this width remains 
constant for 9 mm. anteriorly, greatest convexity of shell 19 mm., of 
pedicle valve 13 mm., of brachial valve 6 mm., delthyrium 6 mm. wide 
at base and 10 mm. high. 
Pedicle valve subpyramidal, with greatest convexity near beak from 
which the surface curves very abruptly anteriorly and laterally to the 
margin of the valve. Beak pointed, very slightly incurved, at times 
slightly twisted. Cardinal area broad, extending to the cardinal extremi- 
ties, almost straight below, more distinctly arched in upper half, vertically 
striate over its entire area. Delthyrium considerably higher than wide 
(its base being about one-half its height), bounded by the thickened mar- 
gins of the strongly developed dental plates; these plates extend about half 
the distance to the frontal margin. Median sinus narrow and smooth at 
beak, becoming broad, of moderate depth, and plicated anteriorly; these 
plications entering in the umbonal region bifurcate rapidly, resulting in 
about 20 low, rounded plications at the front of the valve. Lateral slopes 
of valve each with about 20 low, rounded, simple plications. 
