140 



HELICID<E. 



and fig. 70 b the posterior aspect of both armatures. The palatal 

 armature is also similar to that of P. repercussa, but the first 

 horizontal fold is shorter in the present species correspondingly 

 with the reduction in the upper arm of the bifurcation of the 

 parietal armature, while the vertical lamina is less strong and its 

 edge less thickened. Plectopylis repervussa is, generally speaking, 

 a more solid and larger shell, always lighter in colour than 

 P. bensoni, while its white peristome will at once distinguish it 

 from the latter species. The lower horizontal parietal fold in 

 P. repercussa is always distinctly united to the apertural ridge, 

 whereas in P. bensoni this fold is not visible from the aperture. 

 That these characters are constant, I have reason to believe from 

 having opened sixteen or eighteen specimens without finding any 

 variation in these respects. The specimen shown in figs. 70 a 

 and b measures : major diam. 22, minor 17, alt. 7 mm., while the 



c d 



Fig. 71. Plectopylis bensoni, jur. 



one shown in figs. 70 df measures 27 : 21 : 8 mm. ; both are 

 from Moulmain, and are in my collection. Another specimen in 

 my collection shows no trace of the ridge at the aperture, but is 

 in all other respects like the mature vshells. In figs. 71 a-d 

 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. 71 a), but the palatal armature is quite complete, though 

 correspondingly reduced in size; an earlier set of barriers is 



Fig. 72. Plectopylis bensoni, JUT. 



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. 71 b in situ, while its anterior 

 aspect is given in fig. 71 c the two arrows in fig. 71 d indicate 

 the respective positions of the two sets of barriers. In the 



