Head and body length is 90-135 mm and tail length from 93-139 mm. Weights average from 25-40.8 g. for males and 16.5-25.4 for females. The pelage is made up of different textures, causing an uneven appearance to the soft, thick fur. The coloration tends to be darker above ranging from dark browns to nearly black. The nonprehensile tail is nearly naked and is the same color as the back. The small ears protrude above the fur and the eyes are very small. The head is elongate like that of a true rat or shrew. C. fuliginosus is the best known of all the species. It has five digits on each foot, the forefeet having small outer toes with blunt claws and sharply curved nails on the remaining three. The hind feet have well developed curved claws on all toes but the great toe, which is small with a small nail.

C. fuliginosus is nocturnal and terrestrial. This species lives in the alpine forests and meadows of the Andes. These are cool, wet, heavy vegetation areas, where they can build tunnels in surface vegetation for travel.

They hunt in the early evening and at night; they are mostly nocturnal. They use their long vibrissae (whiskers) and well-developed sense of hearing to locate their prey. They are mainly insectivorous, and can use their incisors to probe in search of insects. However, they are also known to hunt and kill other small vertebrates and earthworms for food. Even small seeds have been found in the digestive tracts.

Breeding season is believed to be early July, as specimens trapped in August were suckling young.

C. fuliginosus is found in the Colombia, NW Venezuela, and Ecuador.