Dwight, J, (1925), The Gulls (Laridae) of the World: their Plumages, Moults, Variations, Relationships and Distribution, Bull. Amer. Mus. Nat. Hist. 52: 63-401.
Dwight describes Larus cachinnans Yellow-legged Gull from p.202-207. Within this form appears to be both Caspian Gull cachinnans and Mediterranean Yellow-legged Gull michahellis as known to today's gull watchers. The pages below contain the summary of Dwight's cachinnans.