Monday, November 07, 2005

The identification of a snail

Here is a snail whos ID I have been working hard at confirming. The two likliest candidates are Discus shimeki, or Discus whitneyi. I am leaning towards D. shimeki, but there are just enough characters that aren't conclusive to keep things interesting. First, here are some photos (The top photo is a different individual, but from the same geographic location and deposit).

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.

1 comment:

  1. Based on Pilsbry, I'd call it D. shimeki, but then again I've never seen D. shimeki.

    The best thing to do would be to compare your shells with reliably identified museum specimens.