1968 Z28 starter woes

[firez]

I tried to start my 68 Z today and the starter would not turn engine over. I pulled it out and used jumper cables and on the floor the starter turned. I put it back in the car with all connections cleaned and tightened up. I turned the key and just had a humm under the hood,but the starter was not kicking in yet it works out of the car on my bench. Any ideas from anyone. Thanks