I've featured Triodopis species here before but I wanted to drop a quick not on identifying the congeners Tridopsis tridentata (Northern Three-tooth) and Triodopsis juxtidens (Atlantic Three-tooth). Later I'll update this article with some comparison photos but here is a mnemonic to keep in mind:
ATlantic is AT or Above,
Northern is southern.
What's being referred to here is which direction the distal edge of the palatal tooth points in relation to the upper palatal tooth-- the key to differentiation (morphologically).
You can read where I gleaned this identification information at the Carnegie site.
The real reason for this article is that I need a device for myself as I think I may be confusing the two. Often. The article says that 'ridge-and-valley' is mostly Atlantic but I've called most Northern. And, earlier today, I was at Frances Slocum and think I may have seen both in close proximity and couldn't remember for the life of me which was which. But maybe there weren't both. I have to be a little more intellectually vigilant sometimes. However, Slocum is just chock full o' Triodopsis so I don't pay that much attention. It's a 'hiding in plain sight' sort of thing.
I'll try to post pics soon to this article to illustrate the point.
UPDATE: No photos yet but I looked through shells I've collected heretofore and they are all indeed T. tridentata. I believe the shells I collected (and live individuals I saw) on the other side of Slocum are T. juxtidens (but I'm in Ithaca right now and don't have them on me) but that could mean both species are present in the park, albeit different sides of the lake. I don't see any appreciable difference in habitat so I find it fairly interesting.