The Collaborative International Dictionary of English v.0.48:

Jacchus \Jac"chus\, n. [NL., fr. L. Jacchus a mystic name of
   Bacchus, Gr. ?.] (Zool.)
   The common marmoset (Hapale vulgaris). Formerly, the name
   was also applied to other species of the same genus.
   [1913 Webster]