http://www.americanhumane.org/assets/pdfs/animals/operational-guides/op-guide-reptilecare.pdf

There's a section on euthanasia.

At risk of starting a discussion that will get this thread pulled, there are a couple of other methods that are perfectly humane but not listed because they are "messy" and not appropriate for a clinical setting (or indoors for that matter). One is probably obvious.
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What goes up must come down...unless it exceeds escape velocity.