Nope - can't see it well enough; would need a close look with a hand lens at least.
But the stone as a whole is fossiliferous, so it could be part of a fossil.
There is an obvious ammonite impression at the top.
The thin line is, I think, a mineral vein. It is standing out because of differential erosion.
You have a multiblock of Dactylioceras sp ammonites.
On the top is a negative (were an ammonite was) underneath is the remanins of another 'Dac' and below your pen is another partial 'Dac'
The thin vein is a mineral that has probably replaced part of the 'Dac's' internal structure.