Syncope bassleri Bassler’s Sheep Frog
Some other names for this species:
Bassler’s Humming Frog
This species was included in the genus Chiasmocleis until a major revision of the Microhylidae family by de Sá et al in 2012.
Santa Cruz Forest Reserve, Loreto, PeruJanuary 18, 2014
Bassler’s Sheep Frog (Syncope bassleri)
This is a very small frog; a full-sized adult like this one is still only about an inch long. Like all the other "sheep frogs" (a.k.a. "narrow-mouthed toads"), they are weird-looking little wedge-headed things.

Here is a complete list of the herps I saw in the wild on my 2014 MT Amazon Expeditions trip.

Online references:
Printed references: