"she does not buy a pet marmoset"

Pan, a satyr and a monkey, Museo dall'Ara Pacis
Creative Commons AttributionPan, a satyr and a monkey, Museo dall'Ara Pacis - Credit: Giovanni Dall'Orto
The choice of the word "marmoset" is surely a mistake on Graves's part. Marmosets are New World monkeys, which neither Claudius nor his mother would ever have seen. Old World monkeys, on the other hand, were not uncommonly kept as pets. Augustus's joke about Claudius's mother breeding the creatures herself gives rise, later in the novel, to a nickname used by Claudius himself.