We present 6.5-meter MMT and 3.5m APO spectrophotometry of 69 H ii regions in 42 low-metallicity emission-line galaxies , selected from the Data Release 7 of the Sloan Digital Sky Survey to have mostly [ O iii ] \lambda 4959/H \beta \la 1 and [ N ii ] \lambda 6583/H \beta \la 0.1 .
The electron temperature-sensitive emission line [ O iii ] \lambda 4363 is detected in 53 H ii regions allowing a direct abundance determination .
The oxygen abundance in the remaining 16 H ii regions is derived using a semi-empirical method .
The oxygen abundance of the galaxies in our sample ranges from 12 + log O/H \sim 7.1 to \sim 7.9 , with 14 H ii regions in 7 galaxies with 12 +log O/H \leq 7.35 .
In 5 of the latter galaxies , the oxygen abundance is derived here for the first time .
Including other known extremely metal-deficient emission-line galaxies from the literature , e.g .
SBS 0335 - 052W , SBS 0335 - 052E and I Zw 18 , we have compiled a sample of the 17 most metal-deficient ( with 12 +log O/H \leq 7.35 ) emission-line galaxies known in the local universe .
There appears to be a metallicity floor at 12 +log O/H \sim 6.9 , suggesting that the matter from which dwarf emission-line galaxies formed was pre-enriched to that level by e.g .
Population III stars .