Here's what your Mr. Lindsay says about these snail shells concerning "A Pliocene Snail": If there had been gaps in the fossil sequence, we would have thought that these were fossils from several different species. If we look at snails alive today, we can find separate species which differ by less than the difference shown in the picture.
Quick quiz: what's another, more reasonable, more rational explanation for the difference in those snail shells other than evolution into different species?