Two species of nudibranch molluscs are described from the northeastern Pacific Ocean on the west coast of North America. These are: a new aeolid species of Cuthona Alder & Hancock, 1855 (family Cuthonidae) from the Oregonian marine zoogeographical province and a new dorid species of Acanthodoris Gray, 1850 (family Onchidorididae) from the Panamic province in the Gulf of California. The internal anatomy and external features of each species are compared with related species. Differences in structure of the radula, reproductive system and other morphological aspects are described. A revision of the family Cuthonidae is included. Evidence is presented to include the genera Trinchesia Ihering, 1879, and Precuthona Odhner, 1929, within the genus Cuthona. Cuthona alpha Baba & Hamatani, 1963, is synonymized with Catriona columbiana (O'Donoghue, 1922). We consider the New World Cuthonidae to be composed of the following five genera: Catriona Winckworth, 1941; Cuthona Alder & Hancock, 1855; Embletonia Alder & Hancock, 1851; Tenellia Costa, 1877; and Tergipes Cuvier, 1805.
