24 
Pedicle valve with a small fold sharp and narrow at the beak, becoming 
wider and lower as it crosses the umbonal region toward the margin. 
Cardinal area narrow, more elevated than that of the brachial valve. 
Delthyrinm triangular, a little higher than wide, partly covered by a short, 
convex plate of the apex. Interior showing small teeth, each supported 
by a delicate dental lamina, the base of which continues as a slightly 
elevated ridge marking the limits of the small, flabellate muscle scar. 
Brachial valve very slightly convex at the umbo, with a narrow but 
definite sinus which widens towards the margin, producing a more sinuate 
anterior margin than in the pedicle valve; foramen wide. The cardinal 
process seen from the outside is a short, wide projection filling up the 
foramen, the interior, however, shows it to be an elongate vertical plate 
along the whole length of the crural cavity. A low, very broad median 
ridge, which widens anteriorly in its lower part, suggests a strengthening 
of the shell on the interior of the sinus rather than a true septum. 
The interior of the shell places it in the Orthis group. It is provisionally 
placed in the genus Plectorthis because of its bifurcating striae, and its 
wide hinge-line with the cardinal areas of the two valves more nearly 
equal than in typical species of the restricted Orthis, but it resembles 
Orthis in the flatness of the brachial valve and in the short apical plate 
over the delthyrium. It differs from either, however, in the very fine 
striation, in the width of its hinge-line, in the incipient fold and sinus and 
the consequent sinuate anterior margin. The ornamentation is suggestive 
of P. triplicatella, but that species is larger and has a different outline. 
It is more closely allied to P. jamesi, from which it differs, however, in the 
more definite fold and sinus, and consequent more sinuate anterior margin, 
and in the somewhat lower cardinal areas and less prominent beaks. 
Horizon and Locality. Ordovician: Beaverfoot. Three-fourths mile 
east of Palliser pass, British Columbia. 
Orthis marshalli n. sp. 
Plate V, figures 1, 2, 3, 4, 5, 6 
Shell plano-convex, width slightly greater than length, average 
specimen length 8 mm., width 10 mm. Cardinal area of brachial valve 
very small, pedicle area erect, somewhat concave, height about one-third 
length. Delthyrium open, narrow, apex slightly rounded rather than 
angular, without the short apical plate frequently seen in species of Orthis. 
Striae simple, strong, sharply defined, twenty-four on one unusually large 
specimen, but averaging about twenty. 
Brachial valve almost fiat, very slightly convex at the beak, with 
a slight sinus-like depression. No good interior has been preserved, but 
the fragments show the usual plate-like cardinal process to be rather 
thick and rounded. The crural plates have been destroyed except for 
the short, stout bases. Septum broad and strong towards the beak, 
tapering sharply, ending abruptly in a point, about the centre of the shell. 
