Easton's 1897 Bible Dictionary:

Quotations
   from the Old Testament in the New, which are very numerous, are
   not made according to any uniform method. When the New Testament
   was written, the Old was not divided, as it now is, into
   chapters and verses, and hence such peculiarities as these: When
   Luke (20:37) refers to Ex. 3:6, he quotes from "Moses at the
   bush", i.e., the section containing the record of Moses at the
   bush. So also Mark (2:26) refers to 1 Sam. 21:1-6, in the words,
   "in the days of Abiathar;" and Paul (Rom. 11:2) refers to 1
   Kings ch. 17-19, in the words, "in Elias", i.e., in the portion
   of the history regarding Elias.

     In general, the New Testament writers quote from the
   Septuagint (q.v.) version of the Old Testament, as it was then
   in common use among the Jews. But it is noticeable that these
   quotations are not made in any uniform manner. Sometimes, e.g.,
   the quotation does not agree literally either with the LXX. or
   the Hebrew text. This occurs in about one hundred instances.
   Sometimes the LXX. is literally quoted (in about ninety
   instances), and sometimes it is corrected or altered in the
   quotations (in over eighty instances).

     Quotations are sometimes made also directly from the Hebrew
   text (Matt. 4:15, 16; John 19:37; 1 Cor. 15:54). Besides the
   quotations made directly, there are found numberless allusions,
   more or less distinct, showing that the minds of the New
   Testament writers were filled with the expressions and ideas as
   well as historical facts recorded in the Old.

     There are in all two hundred and eighty-three direct
   quotations from the Old Testament in the New, but not one clear
   and certain case of quotation from the Apocrypha (q.v.).

     Besides quotations in the New from the Old Testament, there
   are in Paul's writings three quotations from certain Greek
   poets, Acts 17:28; 1 Cor. 15:33; Titus 1:12. These quotations
   are memorials of his early classical education.