The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
85. Masculine are nouns in -or, -ós, -er, -es (gen. -itis), -ex
(gen. -icis): as, color, flós, imber, gurges (gurgitis), vertex
Exceptions are the following: -
a. Feminine are arbor; cós, dós; linter.
b. Neuter are ador, aequor, cor, marmor; ós
(óris); also os
(ossis); cadáver, iter,
túber, úber, vér; and names of plants and
trees in -er: [QUERY] acer, papáver.