SYSTEMATIC ACCOUNT 171 



amount of immersion of the head in that segment and then it would conform to Nordenstam's variety. 

 The general shape of this variety agrees with ' that of the main species ', but its posterior part is ' not 

 so deeply immersed into the first segment of the pereion as is the case in the main species '. 



The extreme similarity between the main species and the variety multitubercidata suggests that the 

 above assumption is correct, especially when the remaining differences described by Nordenstam are 

 reviewed : 



(i) The eyes of M. stebbingi are described as small and black while in Nordenstam's variety they 

 are totally lacking. A very small amount of pigment is present in the eyes of some of the specimens in 

 the Discovery collections, in others it is absent and the eyes are almost invisible. The presence or 

 absence of a small amount of pigment appears to be a variation within the species. 



(2) Nordenstam described nine tubercles on each of the first four pereion segments, whereas in the 

 main species there are only seven. In specimens of the Discovery collections, the same arrangement 

 of tubercles is found, but one pair is very small and may easily have been overlooked by Ohlin, 

 especially as his specimen was only 13 mm., a little over half the length of Nordenstam's. 



(3) In the variety miiltituberculata, the second peduncular joint of the antennule is longer than the 

 combined length of the third one and the flagellum, whereas, according to Nordenstam (1933, p. 112) 

 in the main species it is only as long as the third peduncular joint plus half the length of the flagellum. 

 Ohlin says that the second joint is nearly equal in length to the combined lengths of the third 

 peduncular joint and the flagellum, but figures it considerably longer. In the specimens of the 

 Discovery collections the length of the second joint is less than that of the third and flagellum 

 together (Text-fig. izd). The flagellum bears an extremely short terminal joint similar to that noted 

 by Nordenstam. 



Nordenstam admits that the characters on which he based his new variety may represent individual 

 variations within the species, and I am convinced that this is so. I have therefore included the variety 

 miiltituberculata in the species M. stebbingi. 



There are certain other points which require clearing up in both Ohlin's and Nordenstam's descrip- 

 tions ; both authors state that the antennule is longer than the antenna, yet measurements of the two 

 appendages show that this is not so (see Text-fig. 12a, d). The apparent shortness of the antenna is due 

 to the sharpness of the angle between the second and third peduncular joints. 



Ohlin gives the number of joints of the palp of the maxilliped as three, and Nordenstam says that 

 the maxillipeds of his variety multitubercidata are 'as in the main species'. Actually there are four 

 joints to the palp (Text-fig. i2e), the first one of which is short. 



According to Ohlin the abdomen consists of four free segments and a telson, while Nordenstam 

 describes the abdomen of his variety as being built up of three free segments and a pleotelson. In 

 specimens of the Discovery collections there are four segments and a telson, but the fourth segment 

 is only free laterally (see Text-fig. 13 a of M. kruimeli, in which a similar arrangement occurs). From 

 this figure it can be seen that the last thoracic somite is very small and has much reduced coxal 

 plates; that the first abdominal segment, which is normally hardly visible in a dorsal view of the 

 animal, bears practically no pleural extensions; that these extensions are well developed on the 

 second and third segments, where they are bent downwards laterally at a sharp angle; and that the 

 fourth segment is very narrow, and fused in the mid-dorsal region with the telson, but is free laterally 

 and bears delicate pleural processes. 



Nordenstam's statements concerning the abdomen are extremely confused. On p. 1 1 1 he says that 

 'the first two segments are bent downwards laterally in a sharp angle to the dorsal surface. Their 

 posterior angles are pointed and directed backwards. The third segment is enclosed by the second 

 and the pleotelson; laterally, it is not bent downwards, unlike the other two free segments.' 



