Oh, my hymnal does include the "Old Hundredth"--in several versions. It simply isn't identified as one of the 'doxologies' in any of the several indexes in the hymnal. Your "Old Hundredth," AnnaS, is in both the 1990 Louisville edition and the 1955 Philadelphia edition I have here at home--the lyrics from both are:
All people that on earth do dwell,
Sing to the Lord with cheerful voice;
Him serve with mirth, His praise forth tell,
Come ye before Him and rejoice.
Know that the Lord is God indeed;
Without our aid He did us make;
We are His folk, He doth us feed,
And for His sheep He doth us take.
O enter then His gates with praise;
Approach with joy His courts unto;
Praise, laud, and bless his name always,
For it is seemly so to do.
For why? The Lord our God is good.
His mercy is forever sure;
His truth at all times firmly stood,
And shall from age to age endure.
