IQT7.] E. W. Vredenburg : On Pleurotoma congener. 319 



mantle-fold which originated the structure shifted its position to 

 the next principal thread posteriorly to the one upon which the 

 structure was first developed. Consequently the expansion shifts 

 its position forward (towards the aperture) and at the same time 

 posteriorly from the one main thread to the other, enclosing the 

 next similar structure produced presumably after a temporary 

 arrest of growth. In the other specimen it is observed that the 

 growth commenced on the more posterior of the threads concerned, 

 shifted to the anterior one, and once more meandered back to its 

 original position. As has already been mentioned, the successive 

 expansions fitting inside one another become finally smaller on ap- 

 proaching the aperture till the sinus is obliterated and the outline 

 of the outer lip becomes regularised in this part of the shell ; but, 

 when this happens, it would appear as though the supposed 

 mantle-fold had once m_ore shifted its position still further forward, 

 for now a distinct stromboid sinus appears as an expansion of the 

 spiral thread at the limit of the concave neck and terminal canal, 

 representing what might be the initial stage of a structure similar 

 to the curious expansions above described. These structures evi- 

 dently result from some hitherto unrecorded anatomical peculi- 

 arity. 



Dimensions. 



Height . • 31 mm. 36 mm. 37 mm. 41 mm. 



Thickness .. 11 ,, 13 ,, 14 ,, 17 ,, 



Height of spire . 17 ,, 20 ,, 21 ,,24 ,, 



Height of body- whorl 20 ,, 22 ,, 23 ,, 25 ,, 



Of the specimens, the measurements of which are above 

 tabulated, the first is the smallest of those available, the last the 

 largest. The two others are those exhibiting the peculiar apertural 

 features above described. 



Remarks. — Compared with the original types, the Andaman 

 specimens above described are generally somewhat smaller, with a 

 more evenly conical, less conoidal spire, the apex, in particular, 

 being much more pointed instead of blunted as it is in the origi- 

 nally figured specimen. The circumsutural rim at all stages of 

 growth and the revolving main keels of the body-whorl anteriorly 

 to the sinus-band are also much more distinctly granulated in the 

 case of the Andaman specimens. 



A fossil variety of this shell occurs in the lower and upper 

 Miocene and Pliocene formations of western India, the same 

 variety also occurring fossil in the upper Miocene formations of 

 Java and Sumatra from which it was described by Martin {loc. cit.) 

 as Pleurotoma coronifera, a name pre- employed in the zoological 

 nomenclature by Bellardi for a fossil species from the Miocene of 

 Piedmont. The blunt posterior swelling of the columella, clearly 

 visible in the fossil specimens from India and from Java, has not 

 been observed in any other species of Pletirotoina fossil or recent. 



In all the fossil specimens from India and Java the outer lip 

 is incomplete. Judging from an illustration published by Sacco 



