A REVIEW OF THE MYSIDACEA 39 



type. This does not appear to have been noted in Petalophthalmus 

 previously, though it is present in Hansenomysis. 



The pleopods of the female are all uniramous and conform to the 

 figure given by Faxon (1895) . In the male, however, all the pleopods 

 are biramous and agree closely with those of the male of Hansenomy- 

 sis fyllae, figured by Hansen (1908a, fig. 4). I give herewith figures 

 of the pleopods of the male (figs. 4, / to 4 g). The pleopods of the 

 female consist of a sympod of one joint and an endopod which ap- 

 pears to be 2- jointed, a long basal joint and a short terminal joint. 

 The exopod is absent. In the pleopods of the male the sympod is 

 greatly enlarged compared with that of the female. In the first 

 pleopod of the male the endopod is rather similar to the endopod of 

 the female pleopods except that the terminal joint is more elongate. 

 The exopod is a long appendage with the basal part undivided and 

 the terminal portion composed of about 19 joints. In the remaining 

 pleopods of the male the exopod is slightly longer than the endopod 

 and is similar to the exopod of the first pleopod. The endopods have 

 the terminal part divided into about 10 joints. In the endopods of 

 the male pleopods the form of the female pleopods can be detected 

 and it appears that the only change is in the small terminal portion of 

 the female pleopod. This remains as a single joint in the first pleo- 

 pod of the male but becomes greatly elongated and subdivided into 

 10 joints in the second to the fifth pleopods of the male. 



This condition of the pleopods of the male in the genus Petaloph- 

 thalmus has not previously been noted and, as the specimen from which 

 it is described came from the Bering Sea, a new geographical record, 

 and one far from the other known localities for the species, it might 

 at first be supposed that we are dealing with a new species. I do not 

 accept this interpretation of the facts for two reasons. In the first 

 place, the male specimen agrees in every other detail with the females 

 from California and the Caribbean Sea except for the pleopods. In 

 the second place, I do not think that a fully adult male specimen of 

 the genus has hitherto ever been seen. The Challenger specimen 

 measured 40 mm., it is true, but it appeared to have lacked the second- 

 ary sexual characters of a male, namely, the thickened outer flagellum 

 of the antennule, the multiarticulate flagellum of the antenna, and 

 the genital appendix of the eighth thoracic limbs. The present speci- 

 men measures 48 mm. and has all the above criteria of the adult male. 

 I interpret the matter thus : that Sars' specimen was immature, and 

 that the condition of the pleopods of the specimen must, therefore, 

 be regarded as that of immaturity. The pleopods of the Challenger 

 specimen differ from those here described in the narrowness of the 

 sympod, in the un jointed condition of the exopod, and in the un- 

 jointed condition of the terminal part of the endopod. If I am right 



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