I think it's a one-piece, aka Ashtabula, crank which has no spider, where the chainring slides on the left side arm and is held in place with the stationary cone and a pin cast on the inside of the right crank arm. They were common on old Schwinn Varsities and other models, but I'm not sure if they are interchangeable with other brands. Google Ashtabula and read one of Sheldon Brown's articles in the subject. Cranks with spiders, where bcd is an issue, are commonly referred to as 3-piece cranks (left crank arm, bottom bracket, right crank arm).