Hello Stéphane: long time that I didn't visited this very interesting forum, and I want to contribute something now:
This is a small key to some genera like Marmarina, but you'll have to learn to differentiate with other similar genera. I included in this key the main genera of Cetoniinae of South America that are similar to Marmarina, and possibly in this key maybe have some errors!, anyway I think that this is useful to differentiate to Marmarina from other genera.
NOTE: Other Neotropical Cetoniinae genera: Argyripa, Astrocara, Macrocranius, Tiarocera, Cotinis, Heterocotinis, Balsameda, Jansonella, Amithao, Gymnetina, Hadrosticta ,Guatemalica, Howdenipa, Neocorvicoana (and Corvicoana?), Allorhina, Blaesia, Halffterinitis, Neoscelis, Aemilius. probably I didn't included a genus in this list ?, I'll study it later.
1 Anterior border of prosternum lacks thorn....Hologymnetis
– The anterior border of the prosternum with thorn, variable in length and shape ...... 2
2 Dorsal surface of pronotum and elytra with points small, shallow. ........... 3
– Dorsal surface of pronotum and elytra with points broad and deep, visible without magnification... 4
3 Mesosternal projection very robust and directed ventrally...Gymnetosoma
– Mesosternal projection weak and anteroventral directed..... Gymnetis
4 Apex of elytra dentiform, projected. Protibias tridentate.
Dorsal punctuation more or less scattered.........Hoplopyga
– Apex of elytra angled or rounded, no projected. Protibias bidentate.
Dorsal Punctuation dense, coalescent ................ Marmarina
Schürhoff (1937) wrote a key to Gymnetina subtribu, but now there are many new genera. Then I have a desire to make a key for all Neotropical genera of Cetoniinae. Anyway, I hope for now, that this key may be useful for you to separetly Marmarina genus.
Carlos Aguilar Julio