SCIENCE-GOSSIP. 



147 



ARMATURE OF HELICOID LANDSHELLS 



AND 



NEW SECTIONS OF PLECTOPYLIS- 

 By G. K. Gut>E, F.Z.S. 



{Contimicdfrom p. 77. j 



n~*HE genus Plectopylis was established by Mr. 

 ■*■ Benson in the " Annals and Magazine of 

 Natural History" (3), v., (i860), p. 244, and in the 

 preceding volume of the same publication (3), iv. , 

 (1859), p. 95, he described the external characters of 

 the animal of P. aihatiiia. Mr. Sloliczka, however, 

 was the first to examine some species anatomically 

 (Journ. Asiat. Soc. Bengal, 1871, p. 217), the forms 

 investigated by him being P. achatina, P. cydaspis, 

 P. pinacis, and P. iiiacromphalus. He states that on 

 the whole the form of the body closely resembles that 

 of Clausilia, and that a comparison of the interior 

 organisation of the two genera also indicates their close 

 relation. On comparing the jaw of Plectopylis with 

 that of Clausilia, he found both similar in structure, 

 but the shape different and the transverse sulcation 

 only indicated in the latter genus. Much greater, he 

 continues, is the similarity of the Plectopylis jaw with 

 that of Cylindrella, with the exception that the 

 median projection is wanting in the Cylindrella jaw. 

 The arrangement of the teeth of P. achatina and P. 

 cydaspis he also found to agree with that of 

 Cylindrella in the very small size of the centre tooth, 

 but this was not found to be a constant character. In 

 /'. pinacis the centre tooth was larger and more of a 

 shape similar to that of the lateral teeth, which, how- 

 ever, in all the species he found to retain distinctly 

 the helicoid character. 



The true systematic position of Plectopylis still' 

 seems uncertain. Mr. Pilsbry doulttfully places it in 

 the family Helicidae — between the groups Macroogona 

 and Teleophallogona (Manual of Conch, ix.. Index to 

 the Helicidae, 1895, p. 124). He includes two 

 Chinese groups of uncertain affinities, Traumatophora 

 Oct , 1899.— No. 65. Vol. W. 



and Stegodera, each containing one species, but as 

 nothing is known of their anatomy, and as, moreover, 

 they are devoid of the armature characteristic of 

 Plectopylis, I consider it expedient, for the present, to 

 exclude them. 



The shells of Plectopylis are characterised by a more 

 or less depressed discoid form, with a flat or conical 

 spire and a large open umbilicus (narrow in the section 

 Sykesia), the upper surface is usually sculptured with 

 spiral lines, and the immature shells are hirsute. The 

 aperture is semi-circular or lunate, the peristome 

 somewhat expanded and generally thickened, its ends 

 usually united by an elevated ridge on the parietal 

 callus, which has often an entering fold. The arma- 

 ture consists of a vertical or transverse plate or plates 

 with accessory horizontal or oblique folds on the 

 parietal wal! ; and transverse, horizontal or oblique 

 folds and denticles on the palatal wall. " When the 

 animal retracts into its shell, the passage through the 

 folds is generally found to be filled up with mucous 

 secretion, but the body itself mostly retracts o:ie-half 

 of a whorl further inwards. During hibernati m the 

 aperture is besides closed with the usual caU ireous 

 lamina, as in the other Helicidae." (Stoliczka, lourn. 

 Asiat. Soc. Bengal, 1871, p. 218). 



Mr. Benson noted that Plectopylis achatina was 

 ovo-viviparous, and this was found to be the case with 

 all the species examined by Mr. Stoliczka. One 

 specimen of P. cydaspis he found to contain three 

 well-developed embryos, each consisting of three 

 convolutions, regularly coiled in and enclosed in a 

 thin soft sac of calcareous granules, loosely joined 

 together. I have also observed this fact in a specimen 

 of/", lissochlai/iys in Mr. Fulton's possession. 



