74 Transactions. — Zoology. 



I find that my specimens differ from those described by Messrs. Bate 

 and Westwood in one small point, which I had previously overlooked. In 

 theirs the upper surface of the body " is tuberculated, each tubercle emitting 

 a minute seta at its top." In my specimens the tubercles are not very well 

 marked, and the setae, though certainly very small, are perhaps rather too 

 large to be called minute, as compared with the animal itself. 



I do not, however, consider this difference sufficient to warrant its 

 removal from the European species. 



Genus Plakarthrium, [novum). 



Body much depressed, almost flat. Both antennae having some of the 

 basal joints expanded, flat ; outer antenna with a flagellum. Coxae very 

 largely developed. Last pair of pleopoda biramous, lamellar. 

 Plakarthrium typicum, sp. nov. Plate I., fig. 5. 



First two joints of inner antenna much expanded, first sub-rectangular, 

 second sub-triangular, bearing on its posterior border the third joint, which 

 is small and not expanded and is followed by a very small joint bearing 

 two or three auditory cilia. Outer antenna with peduncle of five joints ; 

 the first two small and cylindrical, the third expanded, triangular, fourth 

 expanded, transverse, fifth cylindrical, followed by a slender many-jointed 

 flagellum reaching to the posterior border of the third thoracic segment. 

 Eyes small, placed in the centres of the two rounded lateral portions of 

 the head. Head transverse, about twice as broad as long, entirely enclosed 

 by the expanded joints of the antennae and by the coxae of the first thoracic 

 segment. Thoracic segments sub-equal in length, the central ones being 

 rather broader than the first and the last. Coxae very large, lamellar, more 

 than half as broad as their segments ; coxa of last thoracic segment reaching 

 nearly to the extremity of the last pair of pleopoda. First two pairs of legs 

 slender, three following pairs short and stout, last two pairs slender, similar 

 to the first two, all ending in strong curved claws. Abdomen sub-rect- 

 angular, showing indications of three segments, the last larger than the 

 first two together ; posterior border concave. Last pair of pleopoda 

 apparently arising right at the posterior end of the abdomen, hasal joint 

 short, flat, about as long as broad, inner branch oblong, inner margin 

 straight, outer branch broader, expanding distally. 



Colour — light-reddish brown, with a few small scattered dots of a darker 

 brown. Length about \ of an inch. 



Hab. Lyttelton Harbour. On stems of a brown seaweed, probably 

 Kcklonia radiata. 



I do not know where this peculiar Isopod should be placed. In some 

 respects it is like Amphoroidea, but it differs very greatly from it in others. 

 As yet I have only found it on one kind of seaweed, probably Ecklonia 



