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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 (a), 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 Lxtomostraca 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 Criaus. 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 Acrocalanus 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 double 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, pls. 6 & Io. 
Acrocalanus longicornis Giesbrecht & Schmeil, 1898, 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. go. — 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. 111. — Stat. 112. — Stat. 118. — Stat. 121. — 
Stat. 125 (day). — Stat. 128. — Stat. 138. — Stat. 141. — Stat. 142. — Stat. 143. — 
Stat. 1442" Stat, 148: — “Stat: 165. == "Stat: 168, Stats 160.9 Stata eee Scale 
185. — Stat. 203 (1500 metres to surface). — Stat. 204. — Stat. 205. — Stat. 210°. — 
Stat. 213. — Stat. 214. — Stat. 2158. — 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 
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