The tooth of the dog that bit you?

This is dog lichen, Peltigera canina, which is at its most noticeable round about now when damp conditions plump up its thalli, the grey leaf-like structures seen here. When dry they curl up like paper. The brown discs at the tip of the thalli are the apothecia or spore capsules.

It's the resemblance of the little white pointed spikes on the underside of the thallus to the fangs of a dog that gives the name and it's no surprise that, according the old 'doctrine of signatures', the lichen was once recommended as a sovereign cure for bite of a mad dog.

However the botanist John Lightfoot writing in 1777 rather debunked the idea: "the lichen has a disagreeable musty taste. Half an ounce of the leaves, dry'd and pulveriszed, and mixed with two drachms of powdered black pepper, compose the once-celebrated Pulvis antilyssus, formerly much recommended by the great Dr. Mead, for the cure of canine madness. This medicine was to be divided in four equal parts, one of which was to be taken by the patient every morning, fasting for four mornings successively, in half a pint of warm cow's milk; after which he was to use the cold bath every morning for a month. It is much to be lamented that the success of this medicine has not always answered the expectation. There are instances where the application has not prevented hydrophobia; and it is even uncertain whether it has been at all instrumental in keeping off that disorder."

It strikes me that, following this routine, if the rabies did not get you then pneumonia probably would.