The specific heat capacities at constant volume are:
\[ (C_V)_{\text{mono}} = \frac{3}{2}R, \quad (C_V)_{\text{dia}} = \frac{5}{2}R. \]
The ratio is:
\[ \frac{(C_V)_{\text{mono}}}{(C_V)_{\text{dia}}} = \frac{\frac{3}{2}R}{\frac{5}{2}R} = \frac{3}{5}. \]
Final Answer: \(3 : 5\).