Cheliped parts of the fossil mud shrimp Callianopsis (Callianassa) clallamensis (Thalassinidea) were first recovered in the mid-nineteenth century from Oligocene marine rocks at Clallam Bay on the northern coast of the Olympic Peninsula, Washington State and described in 1924 as Callianassa clalllamensis. A second species, C. twinensis, was identified in 1926 from cheliped pieces found in rocks of similar age in the Twin Rivers area 20 kilometers southeast of Clallam Bay. Both fossil localities are in the Pysht formation of the Twin River Group. In 1997 these two species were reassigned to Callianopsis, and C. clallamensis is now recognized as the male and C. twinensis as the female of that species. An extensive collection of this fossil thalassinidean at the Burke Museum, University of Washington, particularly in calcite-cemented concretions, and commonly as disassociated parts, contains almost all anatomical hard-parts of the species. Study of this collection has made it possible to render a nearly complete morphological reconstruction of both the male and female of the species and to compare these specimens to the holotype, as well as photographs and original diagrams of varied specimens. This composite confirms sexual dimorphism and close morphological similarity to the extant species Callianopsis goniophthalma that lives at slope depths from the Gulf of Alaska to Baja California, Mexico. Sedimentary and paleontological features indicate that the fossil mud shrimp-bearing strata were deposited in upper slope to bathyal water depths.