PLECTOPYLIS. 141 



McAndrew collection, University Museum of Zoology, Cambridge, 

 is a tablet with three shells, labelled " Nanina lacythis, type, 

 Benson Coll." ; but subsequently altered in pencil to " P. leio- 

 phis (?)." I refer these specimens without hesitation to immature 

 forms of Plectopylis bensoni ; one of them is shown in figs. 72 a-c. 

 This specimen has four and a half whorls completed, and pos- 

 sesses the immature barriers half a whorl from the aperture. 

 The parietal armature is composed of only a crescent- shaped 

 vertical lamina, corresponding to the upper and lower bifurcation 

 of the main horizontal fold (see fig. 72 6), while the palatal 

 armature, as it is seen from the outside through the shell-wall, is 

 shown in fig. 72 c (enlarged) ; there are only three folds, i. e. the 

 second and third horizontal ones, which are very short and de- 

 flected posteriorly, and the vertical fold, with a posterior ridge or 

 support below ; the arrow in fig. 72 a indicates the position of 

 this set of barriers. The specimen measures 11 : 10 : 6 mm. ; 

 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 be- 

 low 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 mm. ; the armature is one-half of a whorl from the aperture, 

 and there are traces of an older set one-quarter of a whorl farther 

 back; the upper and the peripheral keels are provided with a 

 fringe of lacinise. These immature specimens are very interesting 

 and instructive, as they tend to indicate the various stages 

 through which the armatures pass in their evolution from simple 

 to complicated barriers. 



Stoliczka remarks (J.A.S.B. xl (1871), p. 221) that Plecto- 

 pylis achatina [bensoni] 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 speci- 

 mens 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. 



The following varieties may be distinguished. 



Var. repercussoides, Gude. 



Plectopylis achatina, var. repercussoides, Gude, Science Gossip 

 N. s. v, 1899, p. 333. 



This variety is intermediate between typical P. bensoni and 

 P. repercussd) having the contour and the white peristome of the 

 latter, but the armature of the former. It further differs from 

 typical P. bensoni in being angulated above at the periphery and 

 below around the umbilicus, a feature it shares with P. repercussa. 



