Abstract
The third species belonging to Zygastrocarcinus from the Cretaceous of the Western Interior is represented by a single carapace collected from below the Groat Sandstone Bed, Gammon Ferruginous Member, Pierre Shale of Carter County, Montana. Comparison of Zygastrocarcinus cardsmithi n. sp. with the other North American congeners, Z. mendryki (Bishop, 1982) and Z. griesi Bishop, 1983, and with the Pacific Slope species Z. richardsoni (Woodward, 1896) suggests early separation of the Pacific Slope and Western Interior lineage having Z. cardsmithi as a possible ancestor to Z. mendryki and Z. griesi.
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