2 HBLICID^B. 



horizontal fold was the true P. plectostoma, the examination of 

 the types did not bear out this view. All the shells of the Benson 

 collection labelled Darjeeling, which are without doubt the types of 

 the species, belong to the form with the horizontal fold, and this 

 form must, therefore, be taken as the true P. plectostoma. The in- 

 spection of a large series of the two forms enabled me to obtain 

 a fairly accurate idea as to the constancy of both forms and to 

 draw up an amended description, which is now reproduced. 



Shell sinistral, disk-shaped, more or less dark corneous brown, 

 opaque, with a conical spire, deeply but somewhat narrowly 

 umbilicated ; it is composed of seven narrow, closely and regularly 

 coiled whorls, which increase slowly and are a little rounded above 

 and below ; the last whorl scarcely widens near the aperture and 

 shortly descends in front. The shell is radiately plicate arid 

 granulated by coarse spiral sculpture above, and decussated below, 

 while the cuticle is thick and distinctly raised into distant 

 transverse plaits. Five lines of scattered hairs, placed on raised 

 ridges, pass round the whole length of the body-whorl, the first on 

 the periphery, the second a little below it, the third, fourth, and 

 fifth wider apart, the last being close to the umbilical angulation. 



a be 



Fig. 31. Plectopylis plectostoma. 



The aperture is broadly ear-shaped ; the peristome is whitish or 

 rufous, thickened and reflexed, the upper margin widely arcuate ; 

 the raised ridge of the parietal callus is scarcely curved, and not 

 perceptibly separated from the margins of the peristome. The 

 parietal armature consists of a strong vertical plate which gives 

 off anteriorly a strong, obliquely ascending support below and a 

 horizontal fold above, slightly notched at the junction; on the 

 posterior side of the plate are found two minute denticles, one 

 near the upper and one near the lower extremity. A single, very 

 short, free horizontal fold is found below the plate. The palatal 

 armature consists of : first, a thin, short, horizontal fold close to 

 the suture ; secondly, a thin but longer and broader fold opposite 

 the upper extremity of the vertical parietal plate, slightly indented 

 in the middle, with the posterior extremity shortly reflected at an 

 angle of 100 ; thirdly, a similar shortly reflected horizontal fold, 

 notched in the middle, and then suddenly deflected vertically ; 

 fourthly, a short, thin, broad fold, which has posteriorly to it 

 an almost vertically deflected short broad fold ; fifthly, a similar 

 short horizontal fold, which has also posteriorly a short, broad, 



