Selachian describes the members of the Selachii, which sound as if they should be Ancient Greek star-nymphs but which are actually fish which have to make do with cartilage instead of bones: that is, sharks, skates, dogfish and rays.
I was going to make a joke about them living in seas, not lakes (selachian, geddit?) but I discover to my horror that there are fresh water sharks:
bull shark, photo by Terry Goss
Speartooth shark, photo by Bill Harrison
which means that I'm going to be counting my toes if I ever have to paddle across a river, I can tell you.
One last word on the subject of sharks: people killed by sharks every year, on average, fewer than ten; sharks killed by people every year, about a hundred million.
So there's an example of hideous ferocity, isn't it?
Word To Use Today: selachian. This word is Greek and comes from selakhē, a shark, and is related, rather wonderfully, to selas, brightness.