In the search for a natural system, taxonomy should be tested against geographical distribution. Few representatives of southern subfamilies (Romanchellinae, Paralaeospirinae) have crossed the equator, so most northern spirorbids which incubate within their tubes are Circeinae or Spirorbinae.
Spirorbis species are mostly boreal and endemic to either the Atlantic or Pacific, but Spirorbis (Spirorbella) marioni and Spirorbis bidentatus are circumtropical and circumglobal. Circeis and Paradexiospira are circumboreal and boreal-arctic,
but extend south to Brittany, Maine, California and Japan. Januinae and Pileolariinae are mostly tropical, but Jugaria, Bushiella and Protoleodora, all closely related and derived from Pileolaria, are exclusively northern. Probably Jugaria originated in the Atlantic
and went through the Arctic to the Pacific. There it gave rise to Bushiella and Protoleodora, with notable speciation around the Sea of Okhotsk.
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