200 DR. W. E. COLLINGE ON 



also the thickening of the outer rim of the 4th. The distal inner lobe is 

 widely expanded, and the basal joint short. 



The first four segments of the mesosome are almost subequal, and a little 

 longer than the last three. The pleural plates of the 1st segment are directed 

 laterally, and terminate in a blunt point, whereas in /. metallica they are 

 slightly expanded, with the anterior and posterior angles rounded. The 

 coxal plates in /. metallica occupy the whole of the lateral margins of the 

 2nd-7th segments, with the posterior angles of the 5th, 6th, and 7th only 

 produced backwards. In /. Steplienseni they all stand out from the pleuron 

 as more or less triangular plates, attaining their greatest dimensions on the 

 4th segment. The terminal portion of the pleuron in each mesosomatic seg- 

 ment is also different from the condition seen in /. metallica, being rounded 

 in front and drawn out as a short sharp point posteriorly. On the 5th-7th 

 in the middle of the lateral border of the pleuron there is a slight indentation. 



The appendages of the mesosome are much more slender than in I. metal- 

 lica, with the last joint much longer and less robust. These are well shown 

 in the five figures given by Stephensen *. On the sides of the joints there 

 are numerous dentate setse, some with double rows of teeth, and others 

 with teeth on one side only, in addition to numerous plain spines of various 

 lengths. 



The metasomatic segments afford a striking contrast to the condition 

 obtaining in I. metallica, where the first two have their pleural plates 

 laterally rounded, also those of the imperfect third segment, while the long 

 terminal segment usually has a more or less strong median ridge, and the 

 lateral margins gradually converging to a truncate posterior extremity. In 

 all the specimens I have examined this posterior margin is sharply truncate, 

 there being no sign of emargination or lateral extensions. In I. Steplienseni 

 the pleural plates of the segments 1 and 2 and the imperfect 3rd are laterally 

 sharply drawn out, whilst the terminal segment has the lateral margins very 

 slightly curved inwards about its middle, with the posterior margin ter- 

 minating in two small, lateral, rounded point?, with a slight indentation 

 beyond each and a faintly produced median process. These characters are 

 present in all the specimens examined ranging from 6 mm. to 31 mm. in 

 length. 



The figure of the uropod given by Stephensen (op. cit. p. 13, fig. 4, Up.) 

 is of 1. metallica, and not of this species. 



I. Steplienseni is undoubtedly closely allied to /. metallica, Bosc, but per- 

 fectly distinct from it, both males and females being easily recognized from 

 the very young stages onwards. 



* Dcanish OceauograpLical Expedition, 1908--10, vol. ii. D. 1. 1915; pp. 1-53. 



