11 If there be a mistake as to the ground on which possession
is acquired, and which it is wrongly supposed will support usu-
capion, usucapion cannot take place. Thus a man's possession
may be founded on a supposed sale or gift, whereas in point of
fact there has been no sale or gift at all.

12 Long possession which has begun to run in favour of a
deceased person continues to run on in favour of his heir or
praetorian successor, even though he knows that the land
belongs to another person. But if the deceased's possession
had not a lawful inception, it is not available to the heir or
praetorian successor, although ignorant of this. Our consti-
tution has enacted that in usucapion too a similar rule shall be
observed, and that the benefit of the possession shall continue
in favour of the successor. 13 The Emperors Severus and
Antoninus have decided by a rescript that a purchaser too
may reckon as his own the time during which his vendor has
possessed the thing.

14 Finally, it is provided by an edict of the Emperor Marcus
that after an interval of five years a purchaser from the treasury
of property belonging to a third person may repel the owner,
if sued by him, by an exception. But a constitution issued by
Zeno of sacred memory has protected persons who acquire
things from the treasury by purchase, gift, or other title, affording
them complete security from the moment of transfer, and guaran-
teeing their success in any action relating thereto, whether they
be plaintiffs or defendants; while it allows those who claim any
action in respect of such property as owners or pledges to sue
the imperial treasury at any time within four years from the