77 
Right valve nearly flat and much more faintly ribbed than the left, 
with finer strise of nearly equal size. Sinus separating the anterior ear 
from the body of the shell much sharper and deeper than on the left valve. 
Remarks . It will be noted from the description that our specimens 
agree closely with D. occidentalis as described in previously published 
reports, except in the greater proportion of length to height and, conse- 
quently, in the larger angle at which the umbonal slopes diverge from 
the beak. The following table compares the measurements of our forms 
with those of previously published examples of this species as given in 
their descriptions and figures. 
Length 
Height 
Angle 
— 
mm. 
mm. 
degrees 
18 
20 
80-85 
Meek and Worthen, Illinois 
37 
40 
84 
Meek, Nevada 
37 
42 
82-86 
White, Arizona 
37 
42 
82 
White, Indiana 
32 
35 
94 
White, Texas 
31 
38 
85 
Keyes, Missouri 
20 
22 
90 
Girty, Colorado 
30 
23 
100 
The Minnewanka form here discussed 
It will be seen that in all, except the Minnewanka form, the height 
exceeds the length and that in most cases the angle at the beak is less. 
Though certain forms come close to ours in these two features, still it 
seems to be true that the Minnewanka forms emphasize the transverse 
diameter, whereas the others emphasize the height, and likewise that our 
specimens show a pronounced tendency to a much more obtuse angle 
at the beak. It would seem that this reversal of the proportion of length 
to height and this greater angle might be considered as characterizing 
a variety which is here called latiformis . 
Locality and Horizon . The species occurs in the Pennsylvanian and 
Permian from Pennsylvania to Texas and Utah. The variety in the 
Minnewanka region in the Permian of section 1-14 (C). 
Genus, Alloiisma King 
Allorisma albertense n. sp. 
Plate IY, figure 8 a, b, c, d 
Description. Shell large, with a length of 60 mm., the greatest height 
(at umbo) of 28 mm., and a thickness of the combined valves of 20 mm. 
Valves equal, moderately convex, curving regularly from hinge-line to 
base. Beaks depressed, anterior, less than a third of the shell’s length 
from the anterior margin. Hinge-line apparently slightly longer than 
two-thirds the length of the shell. Cardinal margin of each valve inflected 
behind the beaks, forming together a lanceolate depression. A low, incon- 
spicuous umbonal ridge extends from the umbo to the posterior margin; 
the shell between this and the cardinal margin is less convex than towards 
the basal margin. Teeth apparently absent. 
10277—6 
