The genus Marmosa is divided into five species groups with a wide range of sizes and coloration. Head and body lengths range from 85-185 mm, with tail lengths from 90-280 mm. Nearly all forms of the genus have brown or black markings around the eyes. The ears can be lowered by crinkling them down, similar to the furling of a sail. Most species have short velvety fur. M. murina has no pouch, but the tail is strongly prehensile. The number of mammae varies from 9-19, depending on the species, and some have pectoral as well as abdominal mammae.

Most species of Marmosa are forest dwellers, preferring streams or moist habitat. They are generally arboreal and nocturnal. Nests are built of leaves and twigs, though abandoned bird's nests are sometimes used. Marmosa are primarily solitary, hunting and nesting alone.

The diet consists of insects and fruits, but may also contain rodents, lizards and bird eggs.

Litter size is 7-9 offspring with the babies being weaned at 60-70 days. Females reach reproductive maturity at 1 year, and the ability to reproduce declines beyond this age.

M. murina is found in Columbia, Venezuela, Trinidad and Tobago, Guyana, Surinam, French Guiana, Brazil, E Ecuador, E Peru and Bolivia.

The brain weight is 767 g. The brain meaurements are: Length AP left 9.9 mm; Length AP right 9.9 mm; Bitemporal width 11.8 mm; Temporal height left 8.2 mm; Temporal height right 8.2 mm.