NUMERICAL ANALYSIS OF MARINE MOLLUSCAN RANGES ON THE EXTRATROPICAL NORTHEASTERN PACIFIC SHELF1

Abstract

The ranges of shelled Pelecypoda and Gastropoda living on the continental shelf between Point Barrow, Alaska, and Cabo San Lucas, Baja California, have been investigated by computer techniques. Species present within every degree of latitude are compared to those present within every other degree of latitude, and a coefficient of similarity calculated for each latitude pair. Coefficients are then clustered by the weighted pair‐group method. Distributional boundaries considered as provincial boundaries are clearly defined and other distributional regions emerge in the analysis. The species range patterns correspond to those displayed by some older studies based on somewhat different data and appear to be real. They evidently reflect the hydrographic patterns governing temperature regimes on the shelf. The provinces owe their characters to present worldwide marine temperature gradients and also to more local features that serve to make each provincial regime distinctive. As climates have evolved, so have provincial regimes, and ancient forerunners of modern provinces have therefore not had similar thermal attributes.