I'm into the heart of species identification with the snail survey project. As I'm going along, my collaborators Kathryn Perez and Jeff Nekola have put together a great key to upper Mississippi Valley gastropods, but I'm still finding it difficult to key out various species of Vertigo with the key and the accompanying photographs.
So, I did what any good taxonomist would do - look very closely at the snails I have, look closely at the pictures and then make a new key tuned to the characteristics that I can see. In this case, shells of Vertigo are very small - less than 2 mm. And while the overall shape of the shells and the sculpture on the whorls is very important, I find the arrangement of the "teeth" in the aperture most helpful in narrowing down the field. So using Nekola's photographs, I've started to sketch my own images of just the apertures - drawings that emphasize the relative size, shape and placement of these teeth. It's just a draft, and these sketches don't account for all of the variation in some of the different species, but just the exercise of drawing each shell aperture has helped me get a feeling for a more reasonable "gestalt" for each species.