106 HELICID.E. 



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 

 Avith the parietal callus ; it is also much broader. The second 

 horizontal plate is likewise broader and flexuous, xA'hile 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 i^erarcta. 

 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 refei'ence to figs. 49 h-d. The first fold is horizontal, small, 

 and bilobed, close to the suture ; then come t\x'o horizontal folds. 



Fig. 49. — Tlectopylis 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 

 hoi'izontal fold with a small denticle a little above and posterior to 

 it. Fig. 49 b 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, Bhnford. 



Helix (Plectopylis) perarcta, Elaiiford, J. A. S. B. xxxiv, ]865, p. 75 ; 



(-todwin-Austeu, P. Z. S. 1874, pi. 74, tig. 4 (shell and armatm-e) ; 



ibid., op. cit. 1888, p. 243. 

 Helix perarcta, Pfeiffer, Novit. Conch, ser. 1, iii. 1869, p. 502, pi. 108, 



figs. 13-^15 ; Hauloy & Theobald, Conch. Ind. 1870, pi. 13, tig. 3. 

 Helix (Atopa [Plectopylis)) perarcta, Tryon, Man. Conch, ser. 2, iii, 



1887, p. 162, pi. 35," li-s. 91-95. 

 Plectopylis perarcta, Pilsbry, Man. Conch, ser. 2, ix. 1894, p. 146 ; 



Gude, Science Gossip, n. s. iii, 1896, p. 155, tig-. 19. 

 Plectopylis {Chersaecia) perarcta, Gude, op. cit. vi, 1899, p. 148. 



Oricjincd description. — "Shell sinistral, widely umbilicated, discoid, 

 rather thin, white, transversely sinuously striated, with faintly 



