  A musical instrument, the plastic hormonica, is defined here as a birthday
present for Dennis Sullivan, who pioneered and helped popularize the hyperbolic
geometry underlying its construction. This plastic hormonica is based upon the
Farey tesselation of the Poincare disk decorated by its standard osculating
horocycles centered at the rationals. In effect, one taps or holds points of
another tesselation tau with the same decorating horocycles to produce sounds
depending on the fact that the lambda length of e in tau with this decoration
is always an integer. Explicitly, tapping a decorated edge e in tau with lambda
length lambda produces a tone of frequency 440 xi^{lambda-12N}, where xi^{12}=2
and N is some positive integer shift of octave. Another type of tap on edges of
tau is employed to apply flips, which may be equivariant for a Fuchsian group
preserving tau. Sounding the frequency for the edge after an equivariant flip,
one can thereby audibly experience paths in Riemann moduli spaces and listen to
mapping classes. The resulting chords, which arise from an ideal triangle
complementary to tau by sounding the frequency of its frontier edges,
correspond to a generalization of the classical Markoff triples, which are
precisely the chords that arise from the once-punctured torus. In the other
direction, one can query the genera of specified musical pieces.
