a voiceless bilabially post-trilled dental stop
A linguolabial trill [r̼] is not known to be used phonemically, but occurs when blowing a raspberry.
http://en.m.wikipedia.org/wiki/Pirahã_language
http://en.m.wikipedia.org/wiki/Recursion
http://en.m.wikipedia.org/wiki/Three_Spheres_II