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Strophomena sp. 
Plate XIII, figures 2a, b 
Shell medium sized, greatest width along the hinge-line which is 
extended into acute ears at the cardinal extremities. Surface marked by 
fine, uneven, bifurcating strise which show two or more finer ones between 
the coarser ones at the anterior margin. Striations curved toward the 
posterior margin. Dorsal valve moderately convex, the central depression 
marked in the posterior portion and continued, but less distinctly, to the 
anterior margin. Ventral valve known only as an interior cast. Adductor 
muscle scars well defined and subcircular in outline and separated by a 
narrow groove indicating the former position of the median septum. 
Hinge-line shows no teeth. 
Occurs in dolomite where the preservation is not very good. 
Locality and Horizon. Le Gros point, North arm, Great Slave lake, 
North West Territories. Ordovician. 
Strophomena ruga n. sp. 
Plate XIII, figures 4a, b 
Outline sub triangular, greatest width along the hinge-line. Dorsal 
valve concave posteriorly, becoming convex or flattened over the medial 
portion and sharply geniculate at the anterior margin. Muscle scars 
with a distinct outline and very slightly longer than wide. Surface marked 
by zigzag corrugations or wrinkles. Striations only seen on the geniculate 
portion of the anterior margin where they are rather coarse. 
Dorsal valve only known. 
Locality and Horizon. Le Gros point, North arm, Great Slave lake, 
North West Territories. Ordovician. 
Cyrtodonta septentrionis n. sp. 
Plate XIII, figures 5a, b, c 
Shell small, ovate; beaks quite prominent and slightly incurved, 
umbones inconspicuous. Shell thin and marked by concentric lines of 
growth. Hinge-plate strong and quite straight. Cardinal teeth well 
developed, three in each valve. Posterior lateral teeth known only from 
a right valve where they are two in number, strong, with the outer one 
especially elongate and curved and situated near the extremity of the 
hinge-line. Paflial line simple. 
Locality and Horizon. North arm, Great Slave lake, North West 
Territories. Ordovician. 
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