From the Synodal Letter of the Same, to the Gauls (cf. P. Coustant, Letters of the Roman Pontiffs)

Decree I. If a virgin already veiled to Christ, who has professed integrity by public testimony and has received from the priest the veil of blessing after prayer has been poured forth, either has secretly committed incest or, wishing to protect the crime, has imposed the name of husband on the adulterer, taking the members of Christ and making them the members of a prostitute, so that she who had been the spouse of Christ would be called the spouse of a man, in a woman of this sort there are as many charges as there are causes of guilt: the purpose of integrity changed, the veil lost, the first faith depraved and rendered void. What sort and how great a satisfaction is needed here? How great the penance of her who has incurred the destruction of the flesh? ...

Decree II. A girl who has not yet been veiled, but had proposed to remain thus, although she has not been veiled in Christ, nevertheless because she has proposed it and has not been veiled in marriage (or: has been veiled?), the nuptials are called furtive (see the difference from the former case: there it is adultery, here only furtive nuptials), from the fact that she has not observed the precept of heavenly marriage, love hurrying on to the blindness of lust. And for these a time of doing penance must be established.

Decree XII. Concerning him who has married the sister of his wife as wife, in the law of the Old Testament it is written that, for raising up seed for a deceased brother, one ought to marry the wife ... But now the Testament does not allow this to be done, where integrity is treated more fully and chastity is praised while Christ teaches.

Cf. Canon II of Neocaesarea and Canon LXI of Elvira.

Decree XIV. Likewise concerning him who has married the wife of his maternal uncle. It is not lawful to marry the daughter of a maternal uncle, because a similar cause, by generating, is separated and purged through the degrees of an extraneous father; but to go back is not right. For he who has presumed to violate the bed of father or mother, this is not called marriage but fornication.

To one looking at them, it is evident that the former words, which exhibit the title of the decree, do not cohere with the decree. The title speaks of the wife of a maternal uncle, the decree of the daughter of a maternal uncle. Peter Coustant suspects that those former words were moved from their own place and wrongly inserted here. By the same right someone could suspect that one little word had fallen out, so that the whole title would have been concerning him who married the daughter of his maternal uncle as wife. But the reason which Siricius brings forward for his decree, although in certain things a little obscure, persuades me that an error has crept into the following words and that it must be read: It is not lawful to marry the wife of a maternal uncle. For the reason finally returns to this: that it is impious to go back, that is, to be joined to progenitors or to those who are in the place of parents, of which sort are a paternal uncle, a maternal uncle, and their wives. And indeed from this head Siricius argues by an analogous reason, namely because it is impious to violate the bed of father or mother. Therefore I do not doubt to read: To marry the wife of a maternal uncle (that is, her who was the wife of a maternal uncle now dead) is not lawful; but let the judgment belong to fair readers.

Marriage of this sort, forbidden because of affinity, does not stand expressly expressed in the law above cited of Diocletian.