I think Oakleaf is right. I have some older SSS shorts (with the older style of Shelastic chamois) and some Ultra-D's -- the chamois in both shorts is the same shape and has the same channel pattern -- but the Ultra-D chamois is considerably denser.

As mentioned above, the Terry Flex Air chamois (in my 2007 Bellas, anyway) is virtually identical in shape and feel to the older Shelastic chamois.

Between the two Shebeest shorts -- both chamois styles are comfortable for me, but I like the Shelastic slightly better (it's fine for any length ride for me). However, the fabric and construction of the Ultra-D's seems a bit nicer.