Prosobranche - Bellerophina minuta (J. Sowerby)
Here a page for the fossils difficult to see with the naked eye, but not yet microscopic...
this one measures approximately 1,5 millimetre! My average photographs do not allow a catch of sight of quality for the moment.
 
 
real size : 1,5 mm
     
to also see at : http://www.gaultammonite.co.uk/Pages/Gastropoda/Gastropoda_Link.htm