Ivory gull narrower os pubis, and potentially more flexibility in skull kinetic structure. Structurally, it is most similar to the kittiwakes; however, recent genetic analysis based on mtDNA sequences shows that Sabine's gull is the ivory gull's closest relative, followed by the kittiwakes, with Ross's gull and swallow-tailed gull sharing a clade with these species. "Pagophila" is maintained as a unique genus because of the bird’s morphological, behavioral and ecological differences from these species. Colloquial names from Newfoundland include slob gull (from "slob", a local name for drift ice) and ice partridge, from a vague resemblance to a ptarmigan. This species is
Percival Gull 6-cylinder engine was an option. The D.2 variants are known generically as the "Gull Four" (not "Gull IV"). That was despite the Javelin 6-cylinder engine in the Gull Four Mk IIA, and that before the war the Gipsy Major-powered variant was known as the "Gull Major". In 1934, one Gull was modified with cabin doors, revised and shorter glazing, and a faired, single-strut main undercarriage. This version was known as the Gull Four Mk III, (retrospectively P.1D), and those refinements were incorporated in all later Gulls. The final variant was the D.3 "Gull Six", similar to the D.2 "Gull Four