COPEPODA. 41 



ehitin everywhere in the cephalobhorax is almost covered with prickles, and the 

 whole animal is more robust. 



MICEOCALANUS (Sars). 



MiCROCALANUS PUSILLUS. 



(Plate II., fig. 5.) 



Mkrocalanus pusillus, Gr. 0. Sars, Crustac. of Norway, IV. (1903), p. 156. 

 Pseudocalanus pygmmus, Sars, Norwegn. N. Polar Expdn., Vol. V. Crustacea, 1900. 

 „ „ Giesbrecht, ' Belgica ' Eeport, p. 20. 



Sars originally described a small Calanoid, Pseudocalanus pygmseus, from Nansen's 

 Polar Expedition, which he subsequently re-named Microcalanus, and included in the 

 new genus a second and still smaller form of M. pusillus. Giesbrecht described a small 

 Calanoid from the Antarctic seas, which agreed generally with Sars' Polar species, except 

 for very small differences, e.g. the length of the anterior antennae, and the length of the 

 outer marginal spines of the exopodites of the feet. The size varied, mostly from 

 • 7-0 • 75 mm. ; some were even smaller. P. pygmseus, Sars {= M. pygmmus, Sars), is 

 a little longer, viz. ' 86 mm. (Sars). 



The ' Discovery ' collection contains many examples of a very small Calanoid 

 which agrees so closely with Sars' M. pusillus, that I regard them as identical ; and 

 Giesbrecht's Pseudocalanus pygmseus must, I think, be also regarded as identical. This 

 small Calanoid is one of the few examples of complete agreement in form and structure 

 between the Polar and Antarctic forms, and on that ground is of interest. 



The female is 0'60 mm. long. Cephalo thorax two and a half times longer than 

 the abdomen ; the head and first segment united, the former evenly rounded and with 

 short, delicate rostrum ; the greatest breadth not quite half the length ; the last 

 thoracic segment with rounded and only slightly produced margins ; abdomen of four 

 segments. The genital segment is nearly twice as long as the next, which is rather 

 larger than the distal segment, and the anal as long as the segment preceding it. 

 Furcal segments as long as the anal and longer than broad, with four short terminal 

 bristles. The genital segment is very swollen laterally, but symmetrical, and rather 

 tumid ventrally. 



Anterior antennae reaching about the end of the genital segment, and of twenty- 

 three joints. 



Posterior antennae with Re about one-third longer than the Ri. 



Mandibles with Ri much longer than Re, both rami fully segmented. 

 Masticatory plate with strong cutting teeth seven or eight in number, and distally 

 nearly half as broad as long. 



Anterior foot jaws with five well-formed lobes, of Calanus type, with well- 

 segmented Ri. 



