Hemicyclops thalassius nov. spec. (Copepoda, Cyclopoida) from Mar del Plata, with revisionary notes on the family Clausidiidae

INTRODUCTION The discovery of a new species of Hemicyclops, found pelagically in Argentine coastal waters, has made it necessary for us to summarize the descriptions of the species of Hemicyclops Boeck, 1872. In the course of our investigation, the results of which are laid down is this paper, it became necessary to construct a new key for the identification of the genera of Clausidiidae, which will be presented below. We have thought it advisable to state very briefly the position of the genera, basing ourselves mainly on the recent review of this family by Bocquet & Stock (1957). We refrain, at the present stage, from presenting diagnoses of all genera. Many species are commensals or parasites of Invertebrates and the number of known species has considerably increased during the last years, a process which seems far from having come to an end at the present moment. The conceptions of generic units, therefore, are very likely to be unstable for some time to come. CLAUSIDIIDAE Embleton, 1901 The family name Clausidiidae has been suggested by Embleton (1901, 213) to replace the older name Hersiliidae Canu (1888: 792), the preoccupied name of the type genus, Hersilia Philippi (1839: 128), being replaced by Clausidium Kossmann (1874: 11). In this family we have, besides the type genus Clausidium Kossmann, admitted the following genera: Conchyliurus Bocquet & Stock (1957: 215), Giardella Canu (1888a: 409), Hemicyclops Boeck (1872: 42), Hersiliodes Canu (1888a: 417), Hippomolgus G. O. Sars (1917: 147), Leptinogaster Pelseneer (1929: 37), Myzomolgus Bocquet & Stock (1957a: 411), and