[XeTeX] XeLaTeX - Tamil font problems in Ubuntu Linux

[P. P. Narayanaswami]

In Ubuntui Linux, when I run xelatex, with

\def\tam{\fontspec[Script=Tamil]{TAMu_Maduram}}
பல்லவி

no output is produced and I get an error message - "TaMu" not loadable  etc
etc.
It seems the xelatex is only reading the name of the font as "TAMu", and
ignoring the material starting with the underscore.
The same error is happening even if I use any of the Tamil fonts
lohit_ta, TAMu_Katampari, TAMu_Kalyani, TSCu_Times, etc.  All these fonts
are installed in my system (I checked woth fc-list command).
My guess is the "underscore" in the fontname mighty be the problem.
because for Telugu, Malayalam, Devanagari, the following codes work well
and produce the PDF output without errors. (These fontnames have no underscore)

\def\mala{\fontspec[Script=Malayalam]{AnjaliOldLipi}}
\def\skt{\fontspec[Script=Devanagari]{gargi}}
\def\telg{\fontspec[Script=Telugu]{Pothana2000}

How to handle the fonts like TSC_times  or TAM_Kalyani to produce tamil
outpurt?