I have been unable to detect any differences between the specimens regarded as Para- 

 calanus parvus, collected by the 'Siboga', and those taken frequently in plankton samples from 

 the Irish Sea. Wolfenden 1905 («), has described some varieties from J. Stanley Gardiner's 

 Maldive plankton, but whether the minute differences mentioned, are worthy of even making 

 varieties of, is extremely doubtful. In such a widely distributed species as Paracalanus parvus, 

 one would expect to find a certain amount of local variation from the type found in the seas 

 of Northern Europe. 



The species described by my father in the report 'On Entomostraca from the Gulf of 

 Guinea' as Paracalanus pygmaeus, agrees much better with the description and figures of 

 Paracalanus crassirostris Dahl, than with the imperfect description given by Claus. The very 

 strong rostrum, and the short spines on the fifth feet of the female, are two important characters, 

 along with its small size, by which it differs from any of Wolfenden's varieties. 



Paracalanus parvus, has a remarkably wide distribution throughout the great Oceans, 

 as shewn by the record of observers who have noted its occurrence. 



Genus AcrocalariUS Giesbrecht, 1888. 



This genus is closely allied to Paracalanus, but is distinguished from it, by the outer 

 margin of the second and third joints of the exopodites of the third and fourth pairs of swimming 

 feet being armed with teeth. The proximal part of the outer margin of the last joint of the 

 exopodite of the fourth pair of feet, is nearly doublé as long as the distal portion. The fifth 

 feet are very rudimentary, or absent in the female. Only the left fifth foot is present in the male. 



Four species of Acrocalanus were represented in the material collected by the 'Siboga'. 



1. Acrocalanus longicornis Giesbrecht. 



Acrocalanus longicornis Giesbrecht, 1888, p. 332. 



Acrocalanus longicornis Giesbrecht, 1893, p. 171, pis. 6 & 10. 



Acrocalanus longicornis Giesbrecht & Schmeil, 1S9S, p. 25. 



Acrocalanus longicornis Cleve, 1901, p. 5. 



Acrocalanus longicornis Thompson & Scott, 1903, p. 243. 



Acrocalanus longicornis Wolfenden, 1905 (a), p. 1000. 



This species proved to be the most common member of the genus, and was noted in 

 plankton collected at the following stations. 



Stat. 16. — Stat. 35. — Stat. 36. — Stat. 40. — Stat. 47''. — Stat. 50. — Stat. 71. — 

 Stat. 75. - - Stat. 81. - - Stat. 89. -- Stat. 93. -- Stat. 96 (day). -- Stat. 96 (night). - 

 Stat. 98. -- Stat. 99. -- Stat. 110. — Stat. m. -- Stat. 112. —Stat. 118. — Stat. 121.- 

 Stat. 125 (day). - - Stat. 128. — Stat. 138. — Stat. 141. — Stat. 142. -- Stat. 143. - 

 Stat. 144. ■ - Stat. 148. — Stat. 165. - - Stat. 168. -- Stat. 169. -- Stat. 184. -- Stat. 

 185. - - Stat. 203 (1500 metres to surface). - - Stat. 204. — Stat. 205. -- Stat. 2io a . - 

 Stat. 213. - - Stat. 214. -- Stat. 215". -- Stat. 216. -- Stat. 217 (horizontal cylinder). - 

 Stat. 220 (vertical net). -- Stat. 220 (surface). -- Stat. 223. — Stat. 229. -- Stat. 230.- 

 Stat. 243. — Stat. 271. — Stat. 315. 



Acrocalanus longicornis can be easily separated from the other species, by the length 



