Thylamys is a genus of opossum in the family Didelphidae. The premaxillae

Premaxilla

The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

 are rounded rather than pointed. The females lack a pouch

Pouch

Pouch may refer to:* A small bag such as a pocket, teabag, money pouch, sporran, etc* Cadaver pouch, body bag * Diplomatic pouch* Electric heating pouch, medical apparatus, electric heating device for curative treatment...

. The females' nipple

Nipple

In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...

s are arranged in two symmetrical rows on the abdomen All species but T. macrurus store fat in their tails., although this is not necessarily true for all species in the genus.

The genus contains the following species:

• Cinderella Fat-tailed Mouse Opossum
  Cinderella fat-tailed mouse opossum
  The Cinderella fat-tailed mouse opossum is a species of opossum in the family Didelphidae. It is found in northern Argentina and southern Bolivia, in the eastern foothills of the Andes. Its dorsal fur is gray brown to dark brown. Its ventral fur is gray-based, except for the white to yellowish...
  
   (Thylamys cinderella)
• Elegant Fat-tailed Mouse Opossum
  Elegant Fat-tailed Mouse Opossum
  The Elegant Fat-tailed Mouse Opossum is a species of opossum in the family Didelphidae. Its head-and-body length ranges from 106 to 121 millimeters. Its tail length ranges from 115 to 142 millimeters. Its tail is swollen and has fine hairs all over it. Its fur color varies, but it usually ranges...
  
   (Thylamys elegans)
• Karimi's Fat-tailed Mouse Opossum
  Karimi's fat-tailed mouse opossum
  Karimi's fat-tailed mouse opossum is a species of opossum in the family Didelphidae. It is endemic to central and northeast Brazil, where it is found in the cerrado and caatinga. Its head-and-body length is about 95 millimeters, and its tail length is about 72 millimeters. It is very similar to T....
  
   (Thylamys karimii)
• Paraguayan Fat-tailed Mouse Opossum
  Paraguayan Fat-tailed Mouse Opossum
  The Paraguayan Fat-tailed Mouse Opossum is a species of opossum in the family Didelphidae. It is found in forested areas of Brazil and Paraguay. It is known only from a few specimens...
  
   (Thylamys macrurus)
• White-bellied Fat-tailed Mouse Opossum
  White-bellied Fat-tailed Mouse Opossum
  The White-bellied Fat-tailed Mouse Opossum is a species of opossum in the family Didelphidae. It is found in Argentina, Bolivia, Chile and Peru.-Description:...
  
   (Thylamys pallidior)
• Common Fat-tailed Mouse Opossum
  Common Fat-tailed Mouse Opossum
  The Common Fat-tailed Mouse Opossum is a species of opossum in the family Didelphidae. It occurs in Argentina, Bolivia, Brazil, and Paraguay in chaco and Andean foothill habitats. Its head-and-body length is about 75 to 120 mm, and its tail length is about 90 to 134 mm...
  
   (Thylamys pusillus)
• Argentine Fat-tailed Mouse Opossum
  Argentine fat-tailed mouse opossum
  The Argentine fat-tailed mouse opossum is a species of opossum in the family Didelphidae. It is found in the eastern foothills of the Andes in northern Argentina and southern Bolivia. Its dorsal fur is gray brown to dark brown. Its ventral fur is gray-based except for the white to yellowish chest...
  
   (Thylamys sponsorius)
• Tate's Fat-tailed Mouse Opossum
  Tate's fat-tailed mouse opossum
  Tate's fat-tailed mouse opossum is a species of opossum in the family Didelphidae, named after American zoologist George Henry Hamilton Tate. It is found at elevations of 300 to 3,000 m along the coast of central Peru. The species has the northernmost range of any member of its genus. It has white...
  
   (Thylamys tatei)
• Dwarf Fat-tailed Mouse Opossum
  Dwarf Fat-tailed Mouse Opossum
  The Dwarf Fat-tailed Mouse Opossum , also known as the Velvety Fat-tailed Opossum is an opossum species from South America. It is endemic to Brazil, where it is found in cerrado and caatinga habitats. Its head-and-body length is about 141-212 mm, and its tail length is about 73-85 mm...
  
   (Thylamys velutinus)
• Buff-bellied Fat-tailed Mouse Opossum
  Buff-bellied fat-tailed mouse opossum
  The buff-bellied fat-tailed mouse opossum is a species of opossum in the family Didelphidae. It is found in the forests of northern Argentina and southern Bolivia. Its dorsal fur is cinnamon brown...
  
   (Thylamys venustus)

Fossils belonging to the genus date back to the Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

, with the oldest specimens being found in Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

 and Colombia

Colombia

Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...

. Genetic studies indicate that the genus may have originated around 14 million years ago.