The genus Palmoconcha in the family Loxoconchidae of Ostracoda is found in Cenozoic sediments from the North Pacific. The genus Palmoconcha from the North Pacific is divided into two morphological groups: One is the P. parapontica characterized by having a prominent murus along a posterodorsal margin. Another is the P. laevimarginata group characterized by the nonprominent murus. Both the P. parapontica and P. laevimarginata groups contain six species including a new species and six species, respectively. The P. parapontica group is distributed in only northwestern Pacific, whereas the P. laevimarginata group occurs in both the western and eastern sides of the North Pacific. The assemblages occupied by the P. parapontica group, together with Acanthocythereis, Falsobuntonia, and Robertsonites occur from deposits of Japan during the middle Miocene and the early Oligocene climatic cooling. They associate with temperate molluscan faunas, and are considered to have appeared in shelf environments under temperate realms around the Japanese Islands during the Oligocene and Miocene climatic cooling.