Adam Clarke on Romans 3:13

Their throat is an open sepulchre - This and all the following verses to the end of the 18th are found in the Septuagint, but not in the Hebrew text; and it is most evident that it was from this version that the apostle quoted, as the verses cannot be found in any other place with so near an approximation to the apostle's meaning and words. The verses in question, however, are not found in the Alexandrian MS. But they exist in the Vulgate, the Ethiopic, and the Arabic. As the most ancient copies of the Septuagint do not contain these verses, some contend that the apostle has quoted them from different parts of Scripture; and later transcribers of the Septuagint, finding that the 10th, 11th, and 12th, verses were quoted from the xivth Psalm, imagined that the rest were found originally there too, and so incorporated them in their copies, from the apostle's text.