A catena is a form of biblical commentary, verse by verse, made up entirely of excerpts from earlier Biblical commentators, each introduced with the name of the author, and with such minor adjustments of words to allow the whole to form a continuous commentary. The texts are mainly compiled from mainstream authors, but they often contain fragments of certain patristic writings now otherwise lost. It has been asserted by Faulhaber that half of all the commentaries on scripture composed by the Church Fathers are now extant only in this form. - Summary by Wikipedia