In Latin the word "sedens" means "seated,"
More literally "sitting". It's a present participle. Apparently Marsh intended "firmly fixed in its seat".
However, I guessed that "se" could also be the prefix meaning "apart" or "on one's own,"
This prefix is only used on a few verbs. AFAIK it doesn't occur on nouns, and it's rare in general.
while "dens" means "tooth": roughly, one apart from teeth.
If anything, that would rather give "separated tooth", I think... I'm sure it's not a way to form a word like "toothless".
"Without teeth" would be _sine dentibus_.