| abstract | The parameters of the family Halarachnidae are discussed, with diagnoses of the subfamily Halarachninae and the 2 genera, Halarachne and Orthohalarachne, occurring in marine mammals. Halarachne laysanae, n. sp. is described from the Hawaiian Monk Seal, Monachus schauinslandi. Observations based on study of cotypes of Halarachne americana are recorded and the species is differentiated from others in the genus. Observations based on study of Halarachne halichoeri collected from the type-host, Halichoerus grypus, are recorded and compared with those for specimens loaned from various collections, including specimens identified as H. halichoeri by Oudemans and Vitzthum. The species is redefined and differentiated from the others of the genus. Halarachne taita and Halarachne erratica are synonymized under H. halichoeri. A redescription of Halarachne miroungae is given, based on Ferrisʼ type series. Comments on intraspecific variation are based on newly recorded collections. A key is provided to identify females of the 4 species of Halarachne. |