From back in early last summer: a pair of snails, probably Cepaea nemoralis, mating. This species is characterized by the dark brown line round the lip of the open end of the shell. As you can see, the markings on the rest of the shell are pretty variable. Note also the white gypsobelum ('love dart') sticking into the left-hand snail.
Photo taken in the field below Chazey Wood, near Caversham, UK, on 2012-06-09.