1915.] G. K. Gude: Mollusca, IV. 509 



Plectopylis (Chersaecia) williamsoni, sp. n. 



(PI. xlii, figs. I, la, lb, ic, id). 



Shell sinistral, depressed conoid, dark corneous, the earlier 

 whorls lighter and shining, moderately umbilicated, closely pli- 

 cate-striate. Whorls 6, closely coiled, increasing slowly and regu- 

 larly, flattened above, convex at the side, tumid below, the last 

 slightly dilated at the mouth, slowly descending for some distance 

 in front, angulated above and around the umbilicus, subangulate 

 at the periphery. Spire depressed, suture linear, apex obtuse. 

 Aperture oblique, crescent-shaped, peristome white, slightly thick- 

 ened, scarcely reflexed, the margins united by a raised flexuous 

 ridge on the parietal wall, slightly notched at the junctions with 

 the peristome above and below. 



Parietal armature consisting of a low vertical plate, both 

 extremities giving off a slight support posteriorly with a slight 

 denticle behind the upper support; above the vertical plate oc- 

 curs a short slender horizontal fold and on the anterior side are 

 two stout horizontal folds, one very short, in a line with the 

 upper extremity, the other nearly median in position and much 

 longer, terminating at about i mm. from the parietal ridge at the 

 aperture ; below the vertical plate rises a filiform fold running paral- 

 lel with the lower suture and united to the parietal ridge at the 

 aperture. The palatal armature may be said to be in two series ; 

 the anterior set consisting of six horizontal folds, the first short 

 and slender, near the upper suture, the next three longer and 

 stouter, bilobed; the fifth still stouter, and almost bisected, its 

 posterior portion deflexed ; the sixth short and slight, near the 

 lower suture; the posterior series consists of a vertical row of five 

 minute denticles, one each between the second, third, fourth, and 

 fifth folds, and two below the latter; with the exception of the 

 sixth fold, which is placed further forward, all the folds terminate 

 posteriorly in a line with the denticles. 



Major diam. 6, minor 55 mm. ; alt. 3*25 mm. 



Hab.— Abor Hills, exact part not indicated {Oakes). 



Type No. 3087 Bril. Mus. 



This new species, of which 8 specimens were collected, resem- 

 bles a small P. sowerbyi or P. plectostoma in outward appearance, 

 but it is related to P. hrahnia. That species, however, besides 

 being larger, may be readily separated by the fact that the 

 parietal vertical plate gives off anteriorly from its lower extremity 

 a short horizontal fold, while the palatal barriers consist of only 

 four horizontal folds, as against six in the new species; on the 

 other hand, in the posterior series it possesses 13 or 14 denticles 

 and P. williamsoni only 6. 



With this new species is associated the name of Mr. Noel 

 Willamson of the Indian Civil Service, an active explorer on the 

 far Assam Frontier, whose murder in 1911 led to the expedition 

 against the hill tribes. 



