Hi Devendra,

this is the GIRDLED HORN SHELL - Cerithideopsilla cingulata (Gmelin, 1791) which is the validated name. Genus Cerithium is not an accepted genus for this species and genus Cerithidea is a synonym. This is common in backwaters and found to be in thousands leaving a trail line in the silty clay soil. The aperture varies in different specimen. Check the variation in shell sculpture and in aperture.

http://www.gastropods.com/1/Shell_811.shtml

Best 

Deepak

Polymorphism is quite frequent in Molluscs. As used in zoology, polymorphism involves morphs of the phenotype. The general matching of shell colours to background colours seen in terrestrial gastropods implies that perhaps visual predation is an important factor to them. In tropical conditions, niche selection could also be considered; darker morphs, which gain more radiant heat than paler ones, will move to more cryptic sites by seeking the shade. Differences in receiving heat energy could make pale morphs disadvantageous compared with dark ones at low temperatures - but advantageous at high ones. In spatially varied temperature conditions a polymorphism could be generated without predation. 

Hi Devendra,

These snails belong to the genus Pirenella (the Recent species have recently been revised by David Reid - pdf attached, a synonym is Cerithideopsilla). The red circles indicate a shell varix (an inflated part of she shell often linked with a pause in growth). Almost all Pirenella species form a varix about two-thirds of a whorl before they form their final, flared and thickened, outer lip and stop growing. So the specimen on the left is subadult and has about two-thirds of a whorl to grow before it forms its adult outer lip. The shell on the right - though smaller- is just about to form its final adult outer lip and stop growing - as shown by the position of the varix (see illustrations in Reid's monograph). You can use details of the spiral scultpure to identify the numerous species in this genus - but the differences are subtle! Within populations of snails that show determinate growth (that is, those that stop growing and have a distinct adult morphology) one often finds within-population variation in adult size.

And I should add...the reason the varix forms in this constant position relative to the adult aperture in Pirenella (and also a variety of other snails) is probably to help the snail to clamp down onto its substrate very firmly - presumably to help resist predators. See fig. 5L in the Reid paper.

Dear Dr Jonathan,

thanks for the clarity, as of today I was still thinking it is genus Cerithideopsilla! and it is not changed in WoRMS database either..

The Reid monograph has only just been published!

Pirenella conica (Blainville, 1829) and Cassidula sp.