PLECTOPTLIS. Ill 



are important differences in the armature of the two. In the 

 present species the parietal vertical plate is rounded at the top 

 and forms a short ridge posteriorly, while another much smaller 

 ridge is formed at the base, first proceeding a little horizontally 

 and then becoming deflected towards the lower suture (see 

 fig. 52 a) ; the principal horizontal fold begins at a little distance 

 from the vertical plate as in P. Iracliy discus, but it is placed above 

 the middle and therefore nearer the upper suture than in that 

 species, and instead of revolving parallel with the suture, it bends 

 upwards a little and proceeds without interruption as far as its 

 junction with the raised ridge of the parietal callus (see fig. 52 a) 

 at the aperture, whereas in its ally this is interrupted. Differences 

 in the palatal armature will be observed on reference to fig. 52 6, 

 where the inner side of the shell-wall bearing the folds and 

 denticles is shown. The first fold is long and horizontal ; the 

 second is also horizontal and bifurcates as in the other species ; 

 next come two series of three folds each, the anterior ones hori- 

 zontal, the posterior ones smaller and descending obliquely ; and 

 lastly we find a strong broad denticle parallel with and near to 

 the lower suture, with a smaller one posteriorly in a line with it. 

 Fig. 52 c shows the barriers parietal and palatal from the 

 posterior side. All the figures are based on a specimen in 

 Mr. Ponsonby's collection. The Benson collection in the Uni- 

 versity Museum of Zoology, Cambridge, contains four specimens 

 of P. dextrorsa; they measure 17 mm. in diameter. 



100. Plectopylis shanensis, Stoliczka. 



Plectopylis shanensis, Stoliczka, J. A. S. B. xlii, 1873, p. 170. 

 Helix (Plectopylis) shanensis, Hanley & Theobald, Conch. Ind. 1875, 



pi. 149, figs. 8, 9 (shell); Nevill, Hand List, i, 1878, p. 71 



(shawensis). 

 Helix (Atopa (Plectopylis)) shanensis. Try on, Man. Conch, ser. 2, iii, 



1887, p. 162, pi. 36, figs. 10, 11. 



Helix (Plectopylis) trilamellaris, Godwin- Austen, P. Z. S. ]875, p. 43. 

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



Gude, Science Gossip, N. si iv, 1897, p. 36, fig. 48 ; vi, 1899, p. 77, 



fig. 105 (shell and armature). 

 Plectopylis (Chersaecia) shanensis, Gude, op. cit. vii, 1899, p. 148. 



Original description : " PI. testa planorbulari, pallide fusca, 

 apice minutissime exserto, pallido ; anfractibus 6 j, augustis, sutura 

 indistincte margin ata junctis, primis 2| ad tribus minute rugulosis, 

 caeteris transversim oblique striatis atque concentrice obsolete 

 striolatis, ultimo ad peripheriara subrotundato, infra paululum 

 augustiore, ad aperturam modice deflexo; umbilico spatioso, 

 anfractus omnes sutura distincte marginata separates exhibente ; 

 apertura angulum circiter 55 attingentem cum axi formante. 

 peristomate undique expansiusculo atque incrassato, margaritaceo 

 lutescente, circumdata, ad utramque terminationem labii subangu- 

 lati profunde incisa ; labio plicis tribus distinctis iustructo, plica 



