26 
of which is a sigmoid curve which continues on the shell about 3 mm. 
beyond the free end of the crural plates. Crural plates flat, exposing 
their long, thin edges to the opposite valve and standing out from the 
callosity that supports them, forming broad, triangular troughs before 
the abrupt deepening of the dental sockets. 
Area of the pedicle valve higher and more triangular than that of 
the brachial valve. Interior of pedicle valve showing widespread triangular 
teeth, supported by a thickening of the shell at their base. Two, long, 
broad muscle scars extend from the beak to beyond the middle of the shell, 
divided by a ridge, bounded by more or less conspicuous, irregular striations. 
The anterior edge of the scars is truncated in outline. About a quarter 
of the distance from the beak the ridge flattens and a narrow groove is 
formed on either side showing the position of the rather narrow anterior 
muscle scars. 
Externally the outline of D. Columbia somewhat resembles S. 
subquadrata, though the sides and anterior margin are more rounded. 
It can at once be distinguished, however, by its sharper and much more 
simple striae, and by the imbricating ornamentation when it is preserved. 
Interiorly the brachial valve differs in the spear-head shape of the cardinal 
process and in the form of the crural plates and their support. The muscle 
scar of the pedicle valve in D. Columbia is longer than that of D. subquadrata; 
the anterior muscle scar is narrower and proportionately smaller. 
None of the specimens of D. Columbia has attained the gibbous form 
of some of the specimens of D. proavita from Stony mountain, Manitoba; 
some of the less robust specimens of D. proavita , however, are comparable 
in outline, but the Rocky Mountain species has finer, more sharply angular, 
and more numerous striae. Internally none of the brachial valves to 
hand, or none of the illustrations, shows the spearhead cardinal process, 
but that may be due to poor preservation. The shape of the muscle 
scars of the pedicle valve are readily distinguished. 
Horizon and Locality. Richmond: Beaverfoot. From the exposure 
on the west side of the mountain at Stoddart creek, and from Fairmont 
Springs, Windermere district, also from several localities east of Palliser 
pass, Rocky mountains, B.C. 
Dinorthis rockymontana n. sp. 
Plate V, figures 11, 12, 13, 14 
Outline, excluding the hinge, almost circular, width a little greater 
than length. Hinge-line slightly less than half the width of the shell. 
Pedicle valve depressed convex, convexity being evenly distributed except 
for a little more prominence in the umbonal region. Brachial valve 
varying from about the same degree of convexity as the pedicle valve 
to a rather more gibbous form. Greatest convexity in the umbonal 
region, rounding down to almost flat on the wings. A faint, flattened 
area in the middle is suggestive of a sinus and makes a slight wave in the 
outline at the margin. Striae fine, very numerous, flattened on tops, 
