Let me add some thoughts to current answers.
First, on why the salary offer is lower than you were led to believe: the department head, who cannot decide the details of the offers, was likely making a vague guess, possibly based on offers in previous years. (Hopefully s/he was not being intentionally deceptive.) I don't know how the department head indicated to you a salary range--perhaps if you directly asked they had to make a quick guess based on information they didn't have. Or maybe they made a reasonable guess that you misinterpreted. Or maybe the administration is being stingy (e.g., perhaps there have been recent budget cuts, or perhaps there has been a change in adminstration).
Often the department head is on your side, and will be negotating for you so you can be frank with them (hopefully you have some idea of whether this is true or not from your discussions). You should ask why there is a difference between what you were led to believe and the initial offer. In addition to the comparison with your current salary and the industry offer, you should mention you have other academic interviews. Comparing industry salaries to academic salaries is apples and watermelons, so the administration may not be be that moved by your industry offer. However, if you have other interviews set up, it's possible the department head can convince the dean or whoever that you're highly desirable and will be likely to quickly accept if you can get a competitive offer (if this is true, make it clear to the department head).
Otherwise, you should try to defer on making a decision until you have your other interviews. If you can get other academic offers (with a higher salary) before you have to make a decision, it is much easier to negotiate. Also sometime soon (depending on the deadline and time of interviews, maybe now, or maybe at the time of the other interviews) you should let the other interviewing schools know you have a decision with a deadline upcoming so they can try to speed up their decision process about you.
However, given that the initial offer was low in your eyes, there's a good chance that the administration will not be willing to make an offer that is significantly higher, so prepare yourself in case you need to make a difficult decision.