126 HELICID^. 



the British Museum. The species is a very interesting one, 

 forming as it does a connecting link between the two sections 

 Chersoicia and PlectopnUs s. s. On the one hand it resembles 

 P. ponsonhyi in the posterior portion of the palatal armature (see 

 fig. 61 /■), and P. leucochila in the parietal barriers (see fig. 61 e) ; it 

 differs," however, from the other members of this group in having 

 a series of liorizontal folds anteriorly to the vertical palatal lamina. 

 On the other hand this biseriate character of the palatal armature 

 unites it with the group of P. plectostoma. In outward appearance 

 the shell of P. woodtliorpei much resembles P. sliiroiensis but it is 

 much larger. The immature specimen referred to, which has 

 nearly six whorls formed, has the armature incomplete, and is in- 

 structive as possibly throwing some light upon the evolution of 

 these structures. The parietal armature here possesses the two 

 vertical laminae, but the horizontal folds given off by the anterior 

 lamina are very short, being only one-quarter of the length of those 

 in the mature shells ; the thin fold near the lower suture is not 

 compressed into a lamellar fold below the vertical laminae, as is the 

 case in the full-grown shells, and it rises much farther back (see 

 fig. 61 rj, which shows portion of the parietal wall with its armature). 

 Of the palatal armature, only the posterior series of processes is 

 present, the anterior series having still to be formed ; a fact cleai-ly 

 pointing to the more recent origin of the biseriate forms. The 

 vertical lamina is distinctly divided into two subequal portions, in 

 consequence of the indentation in the middle being carried down 

 to the base of the lamina ; the ridge connecting the upper ex- 

 tremity of the vertical lamina with the short horizontal fold above 

 it is absent, but in its stead occui-s near the latter a little denticle, 

 while posteriorly to the upper half of the vertical lamina is found 

 a distinct denticle, corresponding to the slight swelling in the same 

 place, mentioned in the diagnosis (see fig. 61 h, which shows the 

 inside of the palatal wall with its armature). 



109. Plectopylis leucochila, Gude. 



Pk'ctopylis leucochila^, Gude, Science Gossip, N. s. iv, 1898, p. 231, 



tig. 06 ; ibid. p. 264 (leucochila). 

 Plectopylis (s. s.) leucochila, Gude, op. cit. vi, 1899, p. 148. 



Original description : — " Shell sinistral, rather solid, discoid, 

 deeply and perspectively umbilicated, pale yellowish corneous, 

 finely and regularly ribbed, ornamented with minute spiral 

 sculpture. Suture almost linear, spire depressed, apex scarcely 

 raised. AVhorls seven to seven and a half, a little rounded above, 

 rather tumid below, increasing slowly and regularly, the last 

 descending abruptly and rather deeply in front. Aperture 

 round]}' oval ; peristome white, a little thickened and strongly 

 reflexed, the margins a little converging; parietal callus with a 

 slightly raised flexuous ridge, separated from both margins of the 

 peristome by a little notch. Umbilicus deep, widely perspective. 



