In English we have tortoise, turtle, terrapin Why in Greek are all three called χελώνα (χελώνα, θαλάσσια χελώνα, είδος χελώνης)?
A Greek biologist who specializes in this type of animals can give us the Greek names for the various kinds of turtles, but ordinary Greeks do not distinguish the species
There is χελώνα (turtle), θαλάσσια χελώνα (sea turtle), νεροχελώνα, υδρόβια χελώνα (terrapin, water turtle) and the diminuitive χελωνάκι, χελωνίτσα for small turtles. All have the same root because they are all members of the same family (χελωνίδες) and look alike, i suppose.
They can all be called chelonians in English, too. I get the impression that 'turtle' is usually synonymous with 'chelonian' in US English, whereas in other varieties of English it tends to refer only to aquatic or amphibious subsets of chelonians.
This sounds sort of like shell-ona to me. Maybe I am just looking for something that doesn't exist, but is there any linguistic stuff connecting this with shell?