Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Nī̆gris, is (Nī̆gir, Vitr. 8, 2, 6; Nī̆-ger, Mart. Cap. 6, § 673), m., a river in the interior of Africa, now the Joliba or Niger, Plin. 5, 4, 4, § 30; 8, 21, 32, § 77.
Hence, Nī̆grītae, ārum, m., the people living near the Niger, Mel. 1, 4, 3; 3, 10, 4; Plin. 5, 8, 8, § 43.