A REVIEW OF THE MYSIDACEA 75 



The endopod of the fourth pleopod of the male (fig. 19, b) has two 

 strong, smooth, modified setae at the apex, set at a widely divergent 

 angle, which is constant and characteristic; the penultimate joint has 

 a long stout and smooth seta on the outer angle ; the exopod has two 

 modified setae at the apex, one of which is very long and charac- 

 teristically bent about one-third the distance from the base, the other 

 short, less than half the length of the other seta, and smooth. 



Endopod of the uropods (fig. 18, d) distinctly shorter than the exo- 

 pod, with a row of spines extending from the statocyst to the distal 

 end, rather irregular and arranged in groups distally; among them 

 near the distal end are three or four specially long, strong, and promi- 

 nent spines. 



Exopod of the uropods (fig. 18, d) with the proximal joint three 

 times as long as the distal, with eight or nine spines somewhat widely 

 spaced occupying more than half of its outer margin; distal joint one 

 and a half times as long as broad. 



Telson (fig. 18, c) three times as long as broad at the base, of the 

 form and shape characteristic of the genus, apex armed with one pair 

 of long and stout spines between which are three small equal spinules 

 and a pair of plumose setae ; lateral margins armed with three strong 

 spines near the base followed by a short smooth portion and then a 

 more or less continuous row of spines to the distal end, the spines 

 arranged in series, especially distally, where there are about five 

 graded smaller spines between each pair of larger spines. 



The copulatory lobe of the eighth thoracic limbs of the male has 

 a rather special form which is best described by a figure (fig. 18, e). 



Length of adult specimens of both sexes, 8 mm. 



Remarks. — Among the many species of Siriella, this species is most 

 closely allied to S. pacifica Holmes. Indeed females of the two species 

 are difficult, if not impossible, to distinguish from one another, but 

 males are readily separated on examination of the third and fourth 

 pleopods. Hansen (1913b) has given a full description and figures 

 of S. pacifica. On comparing this description and figures with those 

 here given for S. roosevelti, the following differences between the 

 two forms emerge: 



1. Third pleopod of the male. In S. pacifica both rami terminate in 

 a single robust spiniform naked seta. In S. roosevelti both rami 

 terminate in two modified setae, one robust, spiniform, and smooth, 

 the other robust and plumose, much stouter than the plumose setae 

 on the other joints. 



2. Fourth pleopod of the male. Endopod in S. pacifica has two 

 strong spiniform setae at the apex set at an acute angle to one another, 

 one straight and the other slightly bent. In S. roosevelti there are 



