It surprised me that Sizzle (the selector engine jQuery uses) comes with a built-in :nth-child() selector, but lacks an :nth-of-type() selector.
:nth-child()
:nth-of-type()
I can't pretend to know how nth-of-type is implemented, but jQuery does provide a mechanism by which you can create your own custom selector.
The following question deals with custom selectors, and may provide a useful insight to you
What useful custom jQuery selectors have you written?