SCIENCE-GOSSIP. 



115 



the ridge at the aperture, but is in all other 

 respects like the mature shells. In figs. 8ia-^ I 

 have shown an immature shell, received from Mr. 

 J. E, Cooper, of Highgate ; it has only four 

 whorls completed, and is only furnished with 

 the posterior portion of the parietal armature 

 (see fig. 8ifl), but the palatal armature is quite 

 complete, though correspondingly reduced in size ; 

 an earlier set of barriers is found three-quarters of 

 a whorl further back ; the parietal folds of this 

 set have been entirely absorbed, but of the palatal 

 folds there are only three, the second and third 

 horizontal, and the vertical fold ; this is shown in 

 fig. 816 in situ, while its anterior aspect is given in 

 fig. 81C ; the two arrows in fig. 81^ indicate the 



a ii c 



Fig. 82. — Plectopylis achatina juv. 



respective positions of the two sets of barriers. In 

 the McAndrew Collection, University Museum of 

 Zoology, Cambridge, is a tablet with three shells, 

 labelled " Nanina lacy this, type, Benson Coll." ; but 

 subsequently altered in pencil to "P. kiophis (?) " 

 I refer these specimens without hesitation to 

 immature forms of Plectopylis achatina ; one of them 

 is shown in figs. Saa-c. This specimen has four 

 and a-half whorls completed, and possesses the 

 immature barriers half a whorl from the aperture. 

 The parietal armature is composed of only a 

 crescent-shaped vertical plate, corresponding to 

 the upper and lower bifurcation of the main 

 horizontal fold (see fig. Szb), while the palatal 

 armature, as it is seen from the outside through 

 the shell-wall, is shown in fig. 82c (enlarged) ; 

 there are only three folds, i.e. the second and 

 third horizontal ones, which are very short and 

 deflected posteriorly, and the vertical fold, with a 

 posterior ridge or support below ; the arrow in 

 fig. 82a indicates the position of this set of barriers. 

 The specimen measures 11 : 10 : 6 millimetres 

 respectively ; the first three and a-half whorls are 

 ribbed, the last whorl only showing spiral sculpture. 

 The cuticle is plaited transversely, and the whorl 

 is angulated above, at the periphery, and below it ; 

 the periphery showing traces of a fringe of laciniae. 

 Below the aperture are found some traces of 

 another set of barriers. The other two specimens 

 referred to measure 9-5 : 8'5 : 5 millimetres ; the 

 armature is one-half of a whorl from the aperture, 

 and there are traces of an older set one-quarter 

 of a whorl further back ; the upper and the peri- 

 pherial keels are provided with a fringe of laciniae. 

 These immature specimens are very interesting 

 and instractive, as they tend to indicate the 



various stages through which the armatures pass 

 in their evolution from simple to complicated 

 barriers. 



Mr. Stoliczka remarks (Journ. Asiat. Soc. 

 Bengal, xl. (1871), p. 221) that Plectopylis achatina 

 is " extremely common on all the limestone hills 

 about Moulmain. Among thousands of specimens 

 not one dextrorse variety was met with. The larger 

 specimens I have seen measured in the longer 

 diameter 35 millimetres, but specimens of half 

 that size, and even smaller than that, often have 

 all the appearance of being full-grown." As it 

 is so abundant a species, it is not surprising 

 that it is so frequently seen in collections. It is 

 the most variable of all the species of Plectopylis, 

 As none of the many forms has been separated, 

 I venture to name a few of the more prominent 

 varieties. 



Plectopylis achatina var. ohesa {}) (figs, ^la-c) is 

 darker in colour than the type, being of a fuscous 

 chestnut. It is more compressed and distinctly 

 ribbed ; the shell is higher in proportion to the 

 diameter, and the umbilicus is deeper ; the last 

 whorl does not widen suddenly, and the right 

 margin of the peristome is depressed, the aperture 

 being consequently somewhat ear-shaped ; the 

 lower side slopes from the peripherial region to 

 the umbilical angulation. The armature does not 



Fig. 83. — Plectopylis achatina var. obesa. 



differ materially from that of the type, except 

 that the median horizontal parietal fold does not 

 quite reach the apertural ridge. Six specimens 

 were received by me from Miss Linter, five 

 of these being more or less decorticated. 

 {To be continued.) 



[Further forms of this species will be described 

 and figured next month. — Ed. S.-G.] 



(1) Plectopylis achatina var. obesa, n. var. (figs. 83rt-f), 

 differs from the type in being more compressed and higlier 

 in proportion to the diameter ; in the last whorl not widening 

 suddenly at tlie aperture, and in tiie lower side sloping from 

 the periphery to the umbilical angulation ; the right margin 

 of the peristome is depressed ; the umbilicus is deeper, and 

 the horizontal median parietal fold does not quite reach the 

 apertural ridge. The shell is darker in colour and more 

 strongly ribbed. — Major diameter, 19 millimetres; minor 

 diameter, 15 millimetres ; altitude, 7 millimetres. — Habitat, 

 Moulmain, Burma. — Type in my collection. 



