1. The "Epistle of Archbishop Athanasios of Alexandria to Marcellinus on the Psalms" is still commonly found in Orthodox Psalters.
2. The "Hypotheses of Eusebius of Pamphilus" as found in the Codex Alexandrinus Psalter can likely be equated with the Υποθέσεις (Hypotheses)—summary of the contents of the Psalms interpreted theologically—found as one of the two prefatory notices in Eusebius' Commentary on the Psalms (εις τους ψαλμους; available in Migne's Patrologia Graeca, vol. 23; no English translation is available). See Lightfoot, Joseph Barber, "Eusebius of Caesarea (20)," A Dictionary of Christian Biography, Literature, Sects, and Doctrines, Vol. 2, William Smith and Henry Wace, ed., John Murray, London, 1880, pp. 336-337.
3. The Odes in the Codex Alexandrinus Psalter could be understood as a collection of Scriptural prayers (as referenced in the third column of the Codex Alexandrinus Table of Contents page). While this would be correct, the purpose of the collection was to facilitate the use of these prayers in a corporate (liturgical) setting. A comparison can be made to Orthodox Psalters commonly found today as follows:
Codex Alexandrinus Odes
|As found in Orthodox Psalter|
|1st: Ode of Moses in Exodus||1st|
|2nd: Ode of Moses in Deuteronomy||2nd|
|3rd: Prayer of Samuel's mother Hannah||3rd|
|4th: Prayer of Hezekiah||5th|
|5th: Prayer of Jonah||6th|
|6th: Prayer of Habakkuk||4th|
|7th: Prayer of Hezekiah||-|
|8th: Prayer of Manasses||-|
|9th: Prayer of Azariah||7th|
|10th: Hymn of the Fathers (of the Holy Three Children/Youths)||8th|
|11th: Prayer of Mariam the Theotokos||9a|
|12th: Prayer of Symeon||-|
|13th: Prayer of Zacharias||9b|
|14th: Dawn (Early Morning) Hymn [Glory to God in the Highest…]||-|
The change from 14 to 9 odes can be understood as a shift to provide a structured hymn or framework of nine Biblical Odes (aka canticles or songs) for church services (e.g., Matins). Thematically, the Biblical Odes are used to guide the development of content of the odes written for the Menaion (e.g., songs about the saints of the day). Traditionally, verses from both the Biblical Odes in the Psalter and the odes in the Menaion entry for the day are sung in an alternating pattern. This framework is known as a canon (not to be confused with the Canon of Scripture or the canons found in the Rudder!). This change from 14 to 9 odes can thus be dated to the 7th century with St. Andrew of Crete and the Great Canon that is used by the Church during the Great Fast. The canon was further developed in the 8th century by Saints John of Damascus and Cosmas of Jerusalem, and in the 9th century by Saints Joseph the Hymnographer and Theophanes the Branded.
Of those Odes no longer found in the Psalter, it should be recognized that it is not as if they were displaced from use, but, rather, can generally be found fixed within various services. For example, the 12th Ode, the Prayer of Symeon, can be found in Vespers, while the 14th Ode, now generally known as the Doxology (or Great Doxology), is found in Compline and Matins.
4. It is very interesting to note that the 11th Ode of the Codex Alexandrinus Psalter gives Mariam (Mary) the title of Theotokos (which, of course, all Orthodox honor her with). What makes it interesting is that this Bible dates to the time surrounding the 3rd Ecumenical Council—431 A.D.—that put down the heresy of Nestorios (which denied that she was the Mother of God).