What is that bit called and what does it do?

Good evening,

I have a bike, a good start.  The other day attached to the bike came loose and stopped the chain moving. I will describe it below

Below the front derailleur attachment on the seat tube there is a plastic horn shaped attachement.  It would pass very close to the chain either above or below.  I have tried google and bike books but as yet I can't find a reference to it.  It is a specialized allez comp.  It mus have come loose and then caught on the chain but why is it there?  Maybe it's a chain catcher to stop the chain falling onto the bottom bracket if it came off.  I will upload a pic if no one has a clue what i'm on about!


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