PARASITIC COPEPODS — WILSON 531 



Order COPEPODA 



Suborder Caligoida 

 Family CALIGIDAE 

 Genus DYSGAMUS Steenstrup and Liitken, 1861 



DYSGAMUS ATLANTICUS Steenstrup and Lutken 



Plate 29, FiGUiiES 139-148 



Dysffamus atlanticus Steenstkup and Lutken, 1861, p. 368, pi. 4, fig. 8. — Scott, 

 1912, p. 579, figs. 1-4, pi. 13, fig. 13. 



Steenstmp and Liitken established a new genus and species of 

 parasitic copepods upon 10 male specimens captured swimming freely 

 in the plankton of tlie north Atlantic. Scott's specimens wei*e also all 

 males obtained in the plankton of the Atlantic both north and south of 

 the Equator. A second species, Nogagus murrayi^ was described and 

 figured by Brady (1883, p. 136), who failed to recognize it as belonging 

 to the genus DysgaTnus^ a fact discovered by Leigh-Sharpe (1934, p. 

 28). The latter considered a third species, D. arionimU'S (Wilson, 

 1907a, p. 713), to be synonymous with Brady's species, admitting 

 differences that the present author believes may distinguish the two. 

 The fourth known representative of this genus is D. longifurcatv^ 

 (Wilson, 1923, p. 11). 



In every instance the specimens upon which the species were estab- 

 lished were males captured in the surface plankton. In the copepods 

 the characteristics of the female f onii the basis for generic and specific 

 distinctions and the characteristics of the male are regarded as second- 

 ary. As a result the validity of the genus and its four species has 

 remained more or less tentative during the 80 years since the first 

 was established. Now, however, examination of a dozen females 

 taken by Dr. L. Howell Rivero from the skin of a whale shark {Rhine- 

 odon typus) off Habana, Cuba, substantiates the validity of the genus 

 and its type species, D. atlanticus. This discovery also increased the 

 possibility of making valid the other three species by suggesting that 

 further search may reveal the females of some or even all of them. 

 The female allotype and paratypes of this species have been deposited 

 in the ]\Iuseum of Comparative Zoology at Harvard University. 



Female. — Carapace almost circular, slightly wider than long, with 

 a short and narrow lobe at each posterior corner tipped with a small 

 spine. The frontal plates are long and wide with convex anterior and 

 posterior margins ; the lateral lobes are widened posteriorly and par- 

 tially divided at the center. The median posterior lobe is a little less 

 than half the width of the carapace and reaches slightly beyond the 

 tips of the lateral lobes. The fourth segment is free and one-fourth 

 as wide as the carapace, with convex lateral margins. The median 



