I mentioned it briefly in
The Satanic Robert E. Howard Part 1, but part of the development of the popular idea of a "serpent race" or "reptilians" was the
Emerald Tablets of Thoth the Atlantean by Maurice Doreal (real name, Claude Doggins), which began to appear in the 1930s and was apparently largely based on pulp stories like Robert E. Howard's "The Shadow Kingdom."
It is curious to think that the serpent-folk form one of the common links in the Mythos stories of Smith, Lovecraft, and Howard.