275 



SCIENCE-GOSSIP. 



plate, were inclnded under this name, and it became 

 therefore necessary to decide which of the two 

 conformed to Mr. Benson's type, the specimens of 

 which I knew to be in the Museum at Cambridge. 

 Through the kindness of Mr. Harmer I have now 

 been able to examine the type specimens, and I am 

 pleased to have an opportunity of figuring one of 

 them. Although Mr. Benson's reference to the 

 armature in his amended description, " lamina i 

 parietali verticali, simplici, lamellis nullis munita," 

 inclined me to believe that the form without the 

 horizontal fold was the true P. plectostoma, the 

 examination of Mr. Benson's type shells does not 

 bear out this view. All the shells of the Benson 

 collection labelled " Darjeeling," which are without 

 doubt Mr. Benson's types of this species, belong to 

 the form with the horizontal fold, and this form 

 must therefore be taken as the true P. plectostoma. 

 Mr. Fulton obligingly sent me twenty-five specimens 

 of each form for inspection, which, in addition to 

 the specimens in my own and other collections, have 

 enabled me to obtain a fairly accurate idea as to 

 the constancy of both forms, the differences of 

 which will be discussed further on. Plectopylis 

 plectostoma is sinistral, disk-shaped, more or less 

 dark corneous brown, opaque, with a conical spire, 

 deeply but somewhat navro'd'ly 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 and 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. 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 jold 

 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 hori- 

 zontal 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 ex- 

 tremity of the vertical parietal plate, slightly 

 indented in the middle, with the posterior ex- 



tremity shortly reflected at an angle of loo'' ; 

 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, descending fold, a little more oblique than 

 the previous one ; and sixthly, a very short and 

 narrow horizontal fold near the lower suture, 

 situate below the space between the two preceding 

 series. Fig. 3g(f is from one of the type speci- 

 mens ; it measures, major diameter, 9 millimetres ; 

 minor diameter, 8 millimetres ; axis, 5 millimetres. 

 Two other of these specimens measure 8' 5 

 millimetres, and one 8 millimetres in diameter. 

 Fig- 39^1 showing the parietal wall with its 

 armature by itself, and fig. 39^, showing the 

 inside of the outer wall with its palatal folds, 

 are from a specimen in my collection, 

 from the Khasia Hills ; it measures — major 

 diameter, 8'5 millimetres ; minor diameter, 725 

 millimetres; axis, 45 millimetres. The specimens 

 of this form submitted to me by Mr. Fulton, all 

 from the Khasia Hills, range from 8 to g milli- 



Fig. 40. — Plectopylis pUctustoma var. tricarinata. 



metres in diameter. An immature specimen in my 

 collection has the armature complete, as in the 

 full-grown specimens, but the palatal folds are a 

 little shorter ; traces of the previous palatal folds, 

 one quarter of a whorl further back, can distinctly 

 be seen through the shell-wall. 



Plectopylis plectostoma var. tricarinata (i) (figs. 40a 

 and b). A tablet in the McAndrew collection con- 

 tains five specimens, labelled "Plectopylis plectostoma, 

 Bengal, Benson coll.," two of which are distinct 

 from the type and appear to be worthy of a varietal 

 name. Besides being larger and more conical than 

 the type, they are also distinctly keeled at tiie 

 periphery and have three distinct raised ridges on 

 the upper side, revolving as far as the fourth 

 whori. I name this form Plectopylis plectostoma var. 

 tricarinata. The entire shell is shown, enlarged, 

 in fig. 40a, while a portion of the last whorl, more 

 enlarged, is shown in fig. 40&. The armature is 

 identical with that of the type. 



(') Plectopylis plectostoma var. tricarinata, n. var. (figs. 40a 

 and b), difJers from the type in being larger, in having the 

 periphery acutely keeled, and in having three raised ridges 

 between the periphery and the suture, revolving as far as 

 the fourth whorl. — Major diameter, 10 millimetres; minor 

 diameter, g millimetres ; a.xis, 6 millimetres. — Habitat, Bengal. 

 — Type in the McAndrew collection of tlie University 

 Museum of Zoology, Cambridge. 



