The intervals in major thirds tuning with twelve notes per octave are
laid out like this:

 8/5   5/3  16/9  15/8
 5/4   4/3   7/5   3/2
 1/1  16/15  9/8   6/5

With nineteen notes per octave they look like:
  
48/25
14/9   8/5   5/3  12/7  16/9  15/8
 5/4   9/7   4/3   7/5  10/7   3/2
 1/1  25/24 16/15  9/8   7/6   6/5

and the original twelve intervals are no longer evenly spaced:

  /
  /    8/5   5/3    /   16/9  15/8
 5/4    /    4/3   7/5    /    3/2
 1/1    /   16/15  9/8    /    6/5

With thirty-one notes per octave they look like:

88/45
14/9   8/5  13/8   5/3  12/7   7/4  16/9  11/6  15/8  48/25
 5/4   9/7  13/10  4/3  11/8   7/5  10/7  16/11  3/2  20/13
 1/1  45/44 25/24 16/15 12/11  9/8   8/7   7/6   6/5  16/13

and the previous nineteen intervals are no longer evenly spaced:

  /  
14/9   8/5    /    5/3  12/7    /   16/9    /   15/8  48/25
 5/4   9/7    /    4/3    /    7/5  10/7    /    3/2    /
 1/1    /   25/24 16/15   /    9/8    /    7/6   6/5    /