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male. The joints of the antennules are usually indefinite and vary from two to four in number. 

 The antennae, mandibles and maxillipedes are entirely suppressed. The joints of the basiopo- 

 dite of the swimming feet are considerably inflated. The exopodites and endopodites of the 

 first four pairs of feet are three-jointed. The fifth pair of feet is rudimentary and one-jointed. 

 The male antennules are both prehensile. 



The females of this genus are readily identified by the three-jointed abdomen and by 

 the furcal joints being furnished with five or six setae. 



Eight species were obtained from the plankton collected by the 'Siboga' in the Malay 

 Archipelago. Five of the species do not appear to have previously been described. 



i. Monstrilla gracilicauda Giesbrecht. Plate LVII, figs. i and 2. 



Monstrilla gracilicauda Giesbrecht, 1893, p. 586, pi. 46. 

 Monstrilla gracilicauda T. Scott, 1904, p. 245, pis. XIII & XIV. 

 Monstrilla gracilicauda T. Scott, 1906(0), p. 366. 

 Monstrilla gracilicauda van Breemen, 1908, p. 206, fig. 217. 



One female apparently identical with the above species was found in the plankton 

 collected at Station 43, off Pulu Kawassang, Paternoster Islands. 



Length 3,5 mm. 



The antennules are four-jointed and are equal to half the length of the cephalic segment. 

 The abdomen is composed of three segments. The genital segment is sub-cylindrical in shape 

 and is equal to twice the combined length of the next two segments. It is as long as the 

 united length of the next two segments and furcal joints. The segment is marked by a faint 

 suture traversing the middle. The third abdominal segment of the 'Siboga' specimen is very 

 little shorter than the second segment. The furcal joints are about one and a half times longer 

 than the anal segment. Each joint is furnished with one outer marginal seta and five apical 

 setae. The middle apical seta is much shorter than the others (Plate LVII, fig. 1). 



The apex of the fifth pair of feet is rather inflated and is directed at right angles to 

 the body. The apex is furnished with two long setae, and a short one (Plate LVII, fig. 2). 



With the exception of the difference in the length of the third segment of the abdomen 

 the 'Siboga' specimen agrees well with the illustrations given by Giesbrecht. 



Monstrilla gracilicauda has been recorded from the British Coasts and from the 

 Mediterranean. 



2. Monstrilla longicornis I. C. Thompson. Plate LVII, figs. 3 and 4. 



Monstrilla longicornis I. C. Thompson, 1890, p. 119. pi. IV. 



Monstrilla longiranis Giesbrecht, 1893, p. 586, pi. 46. 



Monstrilla longicornis I. C. Thompson, 1896, p. 25. 



Monstrilla longicornis T. Scott, 1902, p. 469, pi. XXV, figs. 3 & 4. 



Monstrilla longicornis T. Scott, 1904, p. 244, pi. XIII, figs. 1 — 7. 



Monstrilla longicornis Pearson, 1906, p. H- 



Monstrilla longicornis T. Scott, igo6(a), p. 365. 



Monstrilla longicornis Farran, 1908, p. 91. 



Monstrilla longicornis van Breemen, 1908, p. 204, fig. 215, 



235 



