8 



with both rami multiarticulate. In the Petalophthalmidae all pleopods are unbranched (according 

 to Faxon the left pleopod of fifth pair in his adult female specimen of Ceratomysis spinosa Fax. 

 "bears a slender lateral branch"), but they increase frequently greatly in length from before 

 backwards, so that the fifth pair are even longer than the sixth abdominal segment. In the 

 Mysidae all pleopods are always reduced but in very different degree. In Archceomysis Czern. 

 and Callomysis Holmes (these two genera are perhaps identical) all pleopods consist of a peduncle 

 with two one-jointed rami marked off by articulations ; in Gastrosaccus Norm. the first pair are 

 built as in Arch&omysis, while the other pairs consist of a single joint. In all other Mysidse 

 the pleopods are one-jointed, generally small or very small, oblong or quite styliform, but in 

 rare cases, especially in Pseudomma parvum Yanh., they increase much in length backwards, 

 so that the fifth pair are even slightly longer than the long sixth abdominal segment. In 

 Paranchialina n. gen. (type : Anchialns augustus G. O. S.) the three anterior pairs and in 

 Anchialina the first pair are normal, nearly styliform, while the remaining pairs are transverse 

 plates broader than long. 



B. Male. In Lophogastrida all pleopods are well developed, subsimilar, with both rami 

 multiarticulate; neither any exopod nor any endopod shows any secondary modification as to 

 length or peculiar equipment with spines or hairs. In the Petalophthalmidae and in many genera 

 of the Mysidae all five pairs are well developed, but the endopod of first pair is always without 

 articulations, at most somewhat more than one-third as long as the exopod {Boreomysis arctica Kr.) 

 and generally much shorter. In forms with both rami multiarticulate in the four posterior pairs 

 the endopod of these pairs and frequently also the endopod of first pair has near or at the 

 base a mostly oblong, flattened protuberance projecting less or more outwards; in Anchialina 

 Norm. and Rhopalophthalmus Illig the protuberance is a thin and sometimes large plate varying 

 much in shape. In the Sirïellinse this protuberance consists in all five pairs of a short stalk with 

 two sausage-shaped branches which are either simple or spirally twisted, the latter being the case 

 in second to fourth pairs of pleopods in the majority of species; these transformed appendices 

 are named pseudobranchiae. 



It is a well-known fact that in numerous other genera of the Mysidae one pair of the 

 pleopods have one of the rami and generally the exopod less or more elongated and often 

 besides much altered as to shape and furniture with hairs, while the other ramus, generally the 

 endopod, and the other pleopods frequently, but not always, are somewhat or very much reduced. 

 In the genus Mysidetes Holt. & Tatt. and in two more aberrant types, Heteromysis Smith and 

 Mysidella G. O. S., all pleopods are rudimentary as in the females, consisting of a single joint. 

 Though all these differences in the male pleopods are well known and have been much used by 

 authors especially as generic characters, I think it may be possible to point out some hitherto 

 scarcely recognised main lines of higher systematic importance. 



In the majority of the Siriellinae, viz. in Hemisiriella n. gen. and in many species of 

 Siriella Dana, the exopod of first pair and both rami of the four other pairs are subsimilar in 

 length and other respects, none of the joints or setae being elongated or showing any special 

 equipment. But in the Asiatic forms of Siriella with the armature along the distal third of the 

 telson very irregular - - small spines being intermingled between larger or much larger spines - 



