The Collaborative International Dictionary of English v.0.48:

Prototheria \Pro`to*the"ri*a\, n. pl. [NL., from Gr. prw^tos
   first + qhri`on, dim. of qh`r beast.] (Zool.)
   Same as Monotremata.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Prototheria
    n 1: echidnas; platypus [syn: Prototheria, subclass
         Prototheria]