(tiré du rapport de Alice E. Wilson Pelecypoda of the Ottawa Formation of the Ottawa-St. Lawrence Lowland,
Bulletin 28 de la Commission géologique du Canada, 1956)
Genotype, Sowteria canadensis (Raymond)
Small, convex and in some cases tumid; shell thick, outline quadrate or subtrapezoidal, a little longer than high; anterior margin almost at right angles to the direction of the ventral margin, making a gentle sigmoid curve slightly concave above and convex below; porterior margin oblique, joining the ventral margin with a sharp curve, the latter margin gently rounded; beak rather prominent, projecting slightly above the hinge, directed forward; umbo prominent, the convexity decreasing but continuing obliquely to the posterior extremity, somewhat flattened on both sides; hinge apparently curved; surface of casts with concentric undulations.
Cardinal teeth, if any, not seen; long posterior lateral teeth present; anterior muscle scar between the beak and the junction of the anterior and ventral margins, posterior scar large and faint.
Sowteria is much smaller than Whitella, more quadrate in outline, Whitella lacks the posterior lateral teeth. Cyrtodonta has lateral teeth but differs in shape, size, and external ornamentation.
Sowteria Canadensis (Raymond)
Whitella canadensis Raymond, Amer. Jour. Sci., 4 ser. 20, 1905, p.373.
Sowteria canadensis (Raymond), Whiteaves, Ottawa Nat. 22, 1908, p.112, Plt. 3, figs. 13-15.
Small to medium size, an average specimen measuring 19 mm. in length, 16 mm. in height, and 13 mm. in thickness (both valves); quadrate to subtrapezoidal outline; beak forming one angle of the outline, projecting slightly above and beyond thehinge thence curving down; umbo a little flattened in the casts, prominent, the decreasing convexity continuing to the posterior extremity, a slight flattening of the convexity on the ventral side, curving more abruptly on the dorsal side; hinge line short, gently curved; ligament, escutcheon, and lunule not seen; a few low concentric undulations on the casts but finer concentric lines visible on small fragments of the outer surface.
Cardinal teeth, if any, not seen; four long lateral teeth of varying length; anterior muscle scar large for the size of the shell, deeply impressed, situated midway between the beak and the junction of the anterior and ventral margins; posterior scar larger and very faint; pallial line not seen.
The specimens seen are all casts except that small fragments of the outer surface are preserved in a few, showing the finer concentric lines. A few are less gibbous than most, the curve to the margin is more flattened, and, as a rule, these specimens are a little larger. It may be that they should be considered as a variety but because the difference is slight and the specimens are poorly preserved, they are retained in the species for the present.
Raymond did not illustrate his Whitella canadensis, though he states that it came from Aylmer. Whiteaves specimens are missing and his illustrations are drawings, considerably idealized to judge from specimens to hand. He, too, specifies the locality as Aylmer. Whiteaves, however, in changing the genus gives it the name Sowteria. In front of the old home of the late Mr. Sowter, a local collector of Aylmer, is an outcrop of sandstone, originally designated as of Chazy age, but now known to be a sandstone layer in the basal Palemia beds. A little south of the above outcrop is a sandstone of undoubted Chazy age. The matrix of one specimen might indicate the Chazy sandstone but all other specimens from the localities listed are certainly from Pamelia beds. For these reasons it is thought that the types may have come from Pamelia beds.
Occurrence. Pamelia beds, localities 24, 33, 57, 85, 101.
Type. Plesiotypes, G.S.C. Nos. 11542, 11542a, Pamelia beds, Skead road, east of the National Research Council Laboratories on the Montreal road, and half-way to the river; and G.S.C. No.11603, the Merivale road, west of Ottawa, respectively.