. Mollusca ... Mollusks. 104 HBLIOIDJE. flattened, apex a little raised. Whorls six to seven, increasing slowly and regularly, flattened above, rounded below, the last angulated above the periphery and round the umbilicus, and descending shortly and abruptly in front. Aperture heart-shaped ; peristome white, scarcely thickened, a little reflected; the mar- gins united by an elevated sinuous ridge on the parietal callus, notched at the lower junction. Umbilicus wide and deep. Parietal wall with a thin vertical plate, strongly deflected posteriorly below, and giving off a short horizontal ridge
. Mollusca ... Mollusks. 104 HBLIOIDJE. flattened, apex a little raised. Whorls six to seven, increasing slowly and regularly, flattened above, rounded below, the last angulated above the periphery and round the umbilicus, and descending shortly and abruptly in front. Aperture heart-shaped ; peristome white, scarcely thickened, a little reflected; the mar- gins united by an elevated sinuous ridge on the parietal callus, notched at the lower junction. Umbilicus wide and deep. Parietal wall with a thin vertical plate, strongly deflected posteriorly below, and giving off a short horizontal ridge at the upper extremity on each side ; a long horizontal flexuous fold rises close to the upper extremity of this plate on the anterior side, descending suddenly at first, then ascending gradually, and after- wards gradually descending, following the deflection of the last whorl, becoming united to the ridge at the aperture; a second, shorter, horizontal fold occurs below this one, rising close to the. Fig. 48.—Plectopylis perrieriB, lower extremity of the vertical plate, proceeding horizontally at first, and then ascending a little; another very thin fold rises below the vertical plate, running parallel to the lower suture as far as the aperture, where it unites with the ridge. Palatal folds, five; the first, rather long and thin, near to and parallel with the suture, with a deep indentation near the posterior extremity, dividing it into two unequal parts ; the second, horizontal, a little deflected posteriorly, with an elongated denticle posteriorly, and a second, smaller, one above the first; the third fold much shorter, strongly curved downwards posteriorly, with a minute denticle posteriorly ; the fourth fold vertical with an obliquely descendiug ridge posteriorly at the upper extremity, and bifurcated at the lower extremity, the anterior arm of the bifurcation the shorter ; a minute denticle occurs near the ridge at the upper extremity and a second one near the middle, b
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Keywords: ., bookauthorg, bookcentury1900, bookdecade1900, booksubjectmollusks
