Boa vs python

[SU_reptile]

I have very silly question.
What are the MORPHOLOGICAL differences between boas and pythons that allow you to tell them apart explicitly.
I know that boa is viviparous, but python is not. However I don't mean that kind of traits. Please help me - I was looking up for such informations in encyclopedia and in some text books that I was using during studies, but I failed to find them.

[Darby]

There aren't a lot of obvious differences - boas typically have smaller scales.

Boas are primary western hemisphere, pythons eastern, but that's not morphological...