What does "bridging convolutions in Ateles" mean?

Context:
Even were the presence of the temporo-occipital, or external
perpendicular, sulcus, a mark of distinction between the higher apes
and man, the value of such a distinctive character would be rendered
very doubtful by the structure of the brain in the platyrhine apes. In
fact, while the temporo-occipital is one of the most constant of sulci
in the catarhine, or Old World, apes, it is never very strongly
developed in the New World apes; it is absent in the smaller
platyrhine; rudimentary in Pithecia;* and more or less obliterated
by bridging convolutions in Ateles.

The Descent of Man