Well, to be exact, they have a little bump about the size of a tennis ball on the top of their spines, and you're not likely to hit it when it's imbedded in a six-foot skull.
11 seconds sound clip from the X Minus 1 - A Gun for Dinosaur by L. Sprague de Camp classic radio drama series.
You can hear this line at 00:08:33 in the radio play.
Quote context
[...]
- Nothing, till the force field is built up.
- What happened to the lights?
- Well, there's no current while we're in transition.
- There's usually a touch of vertigo. I shouldn't worry about it. shoot for?
- Look, where do you shoot for? I mean, with dinosaur. What is the best shot?
- Well, you don't try for his brain, you know. They don't have any.
- Try for the heart. They have big hearts, over a hundred pounds.
- An exploding shell in the heart will slow them down, at least.
- Well, the machine won't work more recently than a hundred thousand years ago.
[...]