100 HELICID^S. 



Lt.-Col. Godwin- Austen in 1874 described and figured Plecto- 

 pylis pseudophis which he considered to be allied to P. perarcta, 

 evidently overlooking its close relationship with P. leiophis, as 

 pointed out by me in 1897 and 1898 when I investigated these 

 forms. The principal points of difference alleged between the 

 two supposed species appear to be the following : in P. leiophis 

 the parietal vertical plate is rounded in outline, in pseudophis it 

 is toothed ; in the former the lower parietal horizontal fold is 

 continuous, in the latter it is interrupted; leiophis has a very 

 short horizontal fold between the long upper and the shorter 

 lower parietal fold, a feature lacking in pseudopliis ; while, lastly, 

 the latter was said to be more elevated in the spire. 1 was at 

 first inclined to regard these differences as specifically valid, more 

 especially as I had not been able to examine the barriers of 

 P. pseudophis when dealing with that species in 1897, and had to 

 rely upon Lt.-Col. Godwin-Austen's figure. In 1898 the Rev. 

 Ashington Bullen, since deceased, favoured me with a specimen 

 which accorded with P. pseudopliis in every respect except that 

 it had an additional short fold between the long upper and the 

 shorter lower horizontal parietal fold, resembling leiophis in that 

 feature, but, as I pointed out at the time, this character appeared 

 to be inconstant in that species. In 1908, however, I received 

 three specimens of Plectopylis from Mr. Robert Cairns, which at 

 first I was inclined to regard as an undescribed form intermediate 

 between the two species. This led me carefully to examine once 

 more all the specimens of leiophis in my collection, which had 

 considerably increased since first I studied these structures. 

 This examination proved to me conclusively that the features 

 differentiating the two alleged species are inconstant, for while 

 some specimens have the elevated spire and the toothed outline 

 of the vertical lamina of P. pseudophis, and possess the short fold 

 between the two other horizontal folds, stated to characterize P.leio- 

 pJiis, others, again, have a depressed spire, although the vertical 

 lamina is toothed in a varying degree, being entire in some. The 

 palatal armature is identical in all the specimens examined. It 

 appears, therefore, that no constant character differentiates 

 P. leiophis and P. pseudopliis and the two must consequently be 

 united under one name. Fig. 46 a represents a specimen, labelled 

 pseudopliis, in the British Museum, while figs. 46 6 and c are 

 copied from Godwin-Austen's original figures. The shell drawn 

 in fig. 44 a is in Mr. Ponsonby's collection, showing the anterior 

 view of the two armatures, 6 represents the posterior aspect of 

 these, while c shows the parietal wall with its vertical lamina and 

 horizontal folds. Finally figs. 45 a and b exhibit the parietal 

 and palatal armatures respectively of the shell I received from 

 the late Eev. Ashington Bullen, which I had referred to 

 P. pseudopliis. 



The armature of P. leiophis may now be described as follows : 



The parietal barriers comprise: a strong vertical lamina, toothed 



or entire, angular above, where it gives off posteriorly an abruptly 



