. Mollusca ... Mollusks. 106 HBLIOID^. of the parietal barriers the principal horizontal lamella is more flexuous, being somewhat raised towards the vertical plate as well as towards the aperture before its final deflection at its junction with the parietal callus; it is also much broader. The second horizontal plate is likewise broader and flexuous, while both are a little further distant from the vertical plate (see tig. 49 a). The vertical plate is smaller and is rounded at its upper extremity, while it is not deflected posteriorly below, as is the case in perarcta. There are, besides, two
. Mollusca ... Mollusks. 106 HBLIOID^. of the parietal barriers the principal horizontal lamella is more flexuous, being somewhat raised towards the vertical plate as well as towards the aperture before its final deflection at its junction with the parietal callus; it is also much broader. The second horizontal plate is likewise broader and flexuous, while both are a little further distant from the vertical plate (see tig. 49 a). The vertical plate is smaller and is rounded at its upper extremity, while it is not deflected posteriorly below, as is the case in perarcta. There are, besides, two small very short ridges given off from the extremities of the vertical plate on its posterior side, and the third horizontal is a little longer, as well as more flexuous. The chief difference, however, is in the palatal barriers, as may be seen on reference to figs. 49 h-d. The first fold is horizontal, small, and bilobed, close to the suture; then come two horizontal folds,. Pig. 49.—Plectop^lis shiroiensis. small but comparatively broad ; next a broad and strong vertical bilobed lamella, giving off on the posterior side two short ridges from the base of the lobes, and finally below this a small but broad horizontal fold with a small denticle a little above and posterior to it. Fig. 49 h shows the barriers from the side of the aperture and fig. 49 d from behind. The specimen figured is in Mr, Ponsonby's collection. 97. Plectopylis perarcta, Blcmfm-d. Helix {Plectopylis) perarcta, Blanford, J. A. S. B. xxxiv, 1865, p. 75 ; Godwin-Austen, P. Z. S. 1874, pi. 74, fig. 4 (shell and armature) ; ibid., op. cit. 1888, p. 248. Helix perarcta, Pfeiffer, Novit. Conch. ser. 1, iii, 1869, p. 502, pi. 108, figs. 13-15; Hanloy & Theobald, Conch. Ind. 1870, pi. 13, fig. 3. Helix (Atopa (Plectopylis)) perarcta, Tryon, Man. Conch, ser. 2, iii, 1887, p. 162, pi. 35, figs. 91-95. Plectopylis perarcta, Pilsbry, Man. Condi, ser. 2, ix, 1894, p. 146; Gude, Science Gossip, n. s. iii, 1896, p. 1
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Keywords: ., bookauthorg, bookcentury1900, bookdecade1900, booksubjectmollusks
