The genus Monopylephorus Levinsen, 1884 is revised, and divided into three genera. Monopylephorus includes rubroniveus, limosus, kermadecensis, parvus, irroratus, aucklandicus, and the new species cuticulatus and evertus. Rhizodrilus Smith, 1900 includes lacteus, africanus, pacificus, lowryi, and arthingtonae, the last two being transferred from Torodrilus Cook, 1970 and Rhyacodrilus Bretscher, 1901 respectively. Peristodrilus gen.nov. contains the single species montanus. Two other species (M. longisetosus and M. frigidus) will be referred to a new genus within the Phallodrilinae rather than the Rhyacodrilinae. The single member of the Branchiurinae, Branchiura sowerbyi Beddard, 1892 is related to both Rhizodrilus and Bothrioneurum Stolc, 1888 and is included with the Rhyacodrilinae. Telmatodrilus multiprostatus does not usually have its spermathecae in IX but in X as normal, and is not related to Rhizodrilus.The diffuse prostates in some Rhyacodrilinae may have groups of cells penetrating the muscle layers of the atria, somewhat resembling the clustered cells of the Telmatodrilinae, but with the cell bodies still forming a continuous atrial covering.The forward relocation of the spermathecae in Rhizodrilus and their absence in Bothrioneurum seems to be related to the development of large copulatory bursae in XI and the occupation of much of X by atria and vasa deferentia.

Thirty-nine species of marine tubificids are reported here for North America, the list being compared to the known world fauna. Nine of the species are new to science, and several other species are redescribed, leading to revisions of genera and subfamilies. Other species remain to be described by colleagues, and it is apparent that a wide diversity of marine tubificids exists around the world and at all depths in the oceans.

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