The four Evangelists were conflated by church fathers with the four living creatures that surrounded the Throne of God in the Book of Revelations. In the vision a single creature bore four heads, one was that of a man, the second that of a lion, the third that of an eagle and the fourth that of an ox. Each of the authors of the Gospels was assigned one of these symbols with the analogy that like the four heads attached to a single creature, the four Gospels expressed a single Truth.
The capital is inscribed with a series of initials, which Favereau interpreted as the opening lines for each gospel.

Biblical source: Rev 4:6-8 and the first line of each gospel.

Northwest View: The ox (Saint Luke) and the lion (Saint Mark). The inscriptions: South: L G I X F D F A West: I E I C F D North: Q Q M C O N East: I P E V V EA

If Favereau is correct, the inscriptions are thus:
South: L[IBER] G[ENERATIONIS] I[ESV] X[PISTI] F[ILII] D[AVID] F[ILII] A[BRAHAM] (Saint Matthew)

West: I[NITIVM] E[VANGELII] I[ESV] C[HRISTI] F[ILII] D[EI] (Saint Mark)

North: Q[VONIAM] Q[VIDEM] M[VLTI] C[ONATI SVNT] O[RDINARE] N[ARRATIONEM] (Saint Luke)

East: I[N] P[RINCIPIO] E[RAT] V[ERBVM ET] V[ERBVM] E[R]A[T] (Saint John)

Translated:

South: The book of the genealogy of Jesus Christ, son of David, son of Abraham. (Matt 1:1).
West: The beginning of the Gospel of Jesus Christ, Son of God. (Mark 1:1).
North:Inasmuch as many have undertaken to compile a narrative ... (Luke 1:1, first part).
East: In the beginning was the Word and the Word was [with God] (John 1:1, first part).

Related capitals:

Revelations Capitals:
Capital 10: Chaining the Devil, Og and Magog
Capital 15: Vision of John the Evangelist
Capital 39: Saint Michael fights the dragon
Capital 51: Evangelists with Symbolic Beast's Heads
Capital 63: Grotesque Bowmen

New Testament Capitals:
Capital 1: Martyrdom fo John the Baptist
Capital 6: Martyrdom fo Stephen
Capital 12: Miracles of Christ
Capital 13: The Good Samaritan
Capital 14: Temptation of Christ
Capital 16: Transfiguration
Capital 17: Deliverance of Peter
Capital 18: Christ's baptism by John
Capital 25: Christ washes the feet of his disciples
Capital 27: Lazarus and the Rich Man
Capital 30: Wedding at Cana
Capital 32: Adoration of the Magi, Massacre of the Innocents
Capital 38: Annunciation and Visitation
Capital 44: Miracles of Peter
Capital 47: Calling the Apostles
Capital 56: Christ and the Samaritan Woman
Capital 61: Annunciation to the Shepherds
Capital 64: Raising of Lazarus
Capital 71: Beatitudes

Non-Narrative Capitals:
Capital 3: Babylonia Magna
Capital 9: Jerusalem Sancta
Capital 51: The 4 Evangelists with Beast Heads
Capital 71: Personifications of the Beatitudes

Revelations 4:6-8:

...⁶And round the throne. on each side of the throne, are four living creatures, full of eyes in front and behind:
⁷the first living creature like a lion, the second living creature like an ox, the third living creature with the face of a man, and the fourth living creature like a flying eagle.
⁸And the four living creatures, each of them with six wings, are full of eyes all round and within, and day and night they never cease to sing, "Holy, holy, holy, is the Lord God Almighty, who was and is and is to come!"

The New Oxford Annotated Bible with the Apocrypha, Revised Standard Version.

Inscriptions based on Robert Favereau, Jean Michaud, and Bernadette Leplant, Corpus des Inscriptions de la France Medievale. Vol. 8, Ariege, Haute-Garonne, Hautes-Pyrenees, Tarn-et-Garonne. Paris: Centre National de la Recherche Scientifique, 1982.