Here is a list of the characters listed for D. whitneyi, taken from the BC Museum's "Living Landscapes" website.
Shell small (width, 6.7 mm), depressed-heliciform; subtranslucent brown or occasionally pale coloured; spire low; whorls 4.5, convex or a little angular (especially in juveniles); suture deep; protoconch without riblets; teleoconch with nearly equally spaced axial riblets, extending onto the base, and fine axial striae; aperture rounded and without denticles; outer lip unthickened; umbilicus rather large, about 33-40% of the width of the shell.
And here is the same for D. shimeki:
Shell small (width to 6.5 mm), more or less depressed-heliciform, subtranslucent, yellowish brown; spire moderately elevated or flattened; whorls 4.5, convex; periphery rounded; protoconch smooth; teleoconch with regular, strong axial riblets with fine axial striae between; riblets rarely extending onto the base and becoming lower and irregular on the last whorl; aperture typically rounded, or more ovate in the flattened form of the species; aperture without denticles; outer lip unthickened; umbilicus rather large, about 30% of the width of the shell.
So what do I have? My snails are about the right size for either, although some of them are a wee-bit larger. Most of them are mostly rounded, some do have a sub-angular periphery. I haven't seen any axial striae between the riblets, but the riblets generally are indistinct along the base - especially on the largest specimens. The biggest reason I'm calling it D. shimeki is because of the umbilicus. The umbilicus is about 33% or less of the total diameter on all specimens. I will have to get my measuring ocular out and confirm this statistically, I bet. Lucky for me I'll be heading back to Madison and the library this week.