We all know that medieval Greek commentaries on the bible were compiled by chaining together extracts from commentaries on the book in question by the Fathers. Often these catenas continue to exist, when the original works are lost. They are therefore a valuable source for retrieving early Christian comments on biblical verses.
But … to string these quotes together, the compilers had to adapt the quotations, if only slightly; they had to add bridging words, tweak tenses. They had to abbreviate, very often. So the question before us is whether we can rely on the quotations.
Eusebius of Caesarea wrote a monster commentary on the Psalms. Unusually, a third of it still exists, preserved in ms. Coislin 44. This means we can compare the original text with the catenas, and get an idea of the value of each. Carmelo Curti wrote an interesting article on this , from which I have translated a couple of passages:
Of the famous “Commentarii in Psalmos” of Eusebius of Caesarea, about a third, Pss. 51-95,3, has been transmitted to us directly in the manuscript Coislin 44, saec. X  and the rest of the work, Pss. 1-50 and 95,4-150, came to us through the catenas, i.e. a path which, as is well-known, is among the least easy for the editor of Christian texts in the Greek language. The importance of the Coislin manuscript does not end in giving us a text genuine, complete and, in principle, correct of one part of the commentary of Eusebius. The manuscript also allows us to determine through appropriate comparisons, the value of those catenas that, together with other fragments of the Eusebian commentaries, contain some passages related to Pss. 51-95,3, i.e. that part attested by Coislin 44. This is the case for two catenary codices, Patmos Monastery St. John 215, saec. XII-XIII and Ambrosianus F 126 sup. century. XIII, deriving independently from a common original and, according to the classification of Karo-Lietzmann, Catena-type XI . Together with fragments of other exegetes of the Psalter, the first one transmits fragments of the commentary of Eusebius on Pss. 78,5-150, the other, fragments of the same comment that referring to Pss. 83,4-150 .
In my study published in 1972, comparing the text of these manuscripts with those witnessed by Coislin 44, I have demonstrated: first, that the compiler of the base catena, from which directly or indirectly our two witnesses derive, used a copy which belonged to the same branch of the tradition as the Coislin manuscript and secondly, that this compiler, while often omitting the comment of entire entries, has worked on the text under his eyes generally by abbreviating …, i.e. removing words or phrases or even whole periods not deemed essential to the meaning …. It follows that from Ps. 95.4 — as has been said, with Ps. 95.3 the Coislin manuscript unfortunately stops — the editor of the Eusebian commentary can be certain that the text given by the two catena codices is usually genuine, though mutilated and spoiled by the omission of words or phrases or even whole sentences in the passages relating to verses for which they have preserved the comment.
By contrast, the contribution of the two catenas for the constitution of the exegesis of the Eusebian text on Pss. 51-95,3 — for this section, as we have said, we are aided by Coislin 44 — is of course not as relevant but still not entirely negligible. They in fact, as we will show in this chapter, in many cases allow us to improve the text offered by the Coislin manuscript, some correcting obvious mistakes, others filling gaps, others attesting variants which may deserve more consideration.
As documentation of what we have stated above, we give some examples. We quote the text of Coislin, which generally corresponds to that reproduced in PG 23, noting the variations between the two catena manuscripts in parentheses. …
In conclusion, for the constitution of the text even in that part of the Eusebian commentary that is preserved in Coislin 44, the manuscripts Ambrosiano F 126 sup. and Patmos S. John Monastery 215 can not be ignored. They in fact, as we believe we have demonstrated, correct obvious errors in Coislin 44, restored to Eusebius words (or phrases) missing in this codex — both attributable to the copyist of the oislin ms. or that of his source –, and also offer alternative readings that are worthy, in some cases, of some attention. The mistakes of Coislin in truth are mostly of the sort that could easily be corrected by the action of a prudent, unhurried editor (but all those mentioned in the course of this chapter are found in the edition of de Montfaucon reproduced in PG 23). It is a different matter for omissions, which are always difficult to divine and are risky to infer in any text and, more importantly, in a text of prose. For these the testimony of the two catenary manuscripts becomes extremely important and irreplaceable.
It is always good to test our theories about what is happening in catenas. It is a relief to learn that they really do have value to the editor. That lesson should be applicable well beyond the specific case of Eusebius on the Psalms.
1. C. Curti, I “Commentarii in Psalmos” di Eusebio di Cesarea: tradizione diretta (Coislin 44) e tradizione catenaria. In: Eusebiana 1, 2nd ed, 169-179.