PLECTOPYLIS. 135 



microscopic spiral lines. The spire is slightly conical, the apex 

 scarcely produced, arid the suture linear. Whorls 6, increasing 

 slowly and regularly, a little flattened above and rounded below; 

 the last slightly angulateci above the periphery and around the 

 umbilicus, and descending rather abruptly and deeply in front. 

 The aperture is oblique, heart-shaped. The peristoaie is white, 

 thickened and strongly reflexed ; its margins are united by a 

 strong flexuous raised ridge on the parietal callus. 



The parietal armature is composed of a slight median hori- 

 zontal fold, which proceeds from the apertural ridge, is interrupted 

 for a short distance and then continues parallel with the suture 

 for about a quarter of the last whorl ; it then gives off a shortly 

 descending, slightly reflexed arm, which is provided anteriorly at 

 the lower extremity with a short horizontal ridge ; the fold then 



a b 



Fig. 67. Plectopylis linteree. 



rises obliquely for a short distance and finally bifurcates ; the 

 lower arm of the bifurcation is the longer, and descends obliquely, 

 its lower extremity being provided posteriorly with a short 

 horizontal ridge; the upper arm at first continues to ascend 

 obliquely, then deflects horizontally close to the suture ; a short, 

 free, thin, horizontal fold occurs below the two lower arms, not 

 extending beyond on either side (see fig. 68 d, which shows the 

 parietal wall with its folds). 



The palatal armature consists of : first, a thin long horizontal 

 fold near the suture and parallel with it ; secondly, a shorter but 

 stronger broad horizontal fold, which deflects a little and is 

 slightly indented posteriorly ; thirdly, a still shorter, broad, 

 straight horizontal fold ; fourthly, a strong broad vertical lamina 

 \vhich intercalates between the two lower arms of the parietal 

 fold ; this lamina is inclined towards the aperture, and its edge is 

 thickened and reflexed ; near its lower extremity on the posterior 

 side occurs a strong little denticle, which is elongated horizontally ; 

 fifthly, a short thin horizontal fold close to the lower suture, 

 having an elongated denticle a little above its posterior extremity. 

 The species is closely allied to Plectopylis bensoni, but the spire 

 of the present shell is much more raised, the umbilicus is much 

 deeper, and the whorls more rounded. In the armature this 

 species further differs from P. bensoni in the median parietal fold 

 being interrupted and much slighter, the branched portion being 

 relatively much more elevated ; the lower free horizontal parietal 

 fold is very short, so that this part of the armature, while differing 

 from the typical forms of P. bensoni, recalls the condition which 



