74 Transactions.—Z oology. 
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 sete, 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 antenne having some of the 
basal joints expanded, flat; outer antenna with a flagellum. Cox 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 thoracie 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 antenne and by the coxe of the first thoracic 
segment. Thoracic segments sub-equal in length, the central ones being 
rather broader than the first and the last. Coxe 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 indieations 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, basal joint 
short, flat, about as long as broad, inner branch oblong, inner margin 
"ERR outer branch broader, expanding distally. 
` Colour—light-reddish brown, with a few anal scattered dots of a darker 
brown. Length about 1 of an inch. 
Hab. Lyttelton Harbour. On stems of a brown seaweed, probably 
Ecklonia 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 
