DESCRIPTION OF SPECIES 317 



exopod is about two-thirds the length of the endopod, and both are rather narrow and 

 fringed with plumose setae. 



Distribution. South Sandwich Islands. 



Since the three species S. convexa, Cunningham, S.gaudichaiidii, Aud. et Edw. and 

 S. laevis, Richardson, resemble one another rather closely, a short comparison of the 

 three may prove to be of value. 



(i) The shape of the body differs in the three species; that of S. convexa is pear- 

 shaped, owing largely to the greater length of the terminal segment, and to the fact that 

 the sixth and seventh thoracic somites, with their coxal plates, are distinctly narrower 

 than the preceding ones. This decrease in width is also characteristic of S. laevis; but 

 in this species, which is not as large as the other two, the terminal segment is ovate. In 

 S. gaudichaudii the terminal segment is oval, with the sixth and seventh thoracic somites 

 not distinctly narrower than the preceding ones. 



(2) Small median dorsal spines are present on the fourth and fifth thoracic somites 

 and on the first, second and third abdominal segments of S. convexa, whereas in the 

 other two species they are only present on the abdominal segments. 



(3) The terminal segment of S. convexa is roughly five-sided, with three well- 

 developed dorsal carinae: a median one, interrupted in the middle, and two lateral ones, 

 one on either side of the segment, each terminating in a sharp spine. The terminal seg- 

 ment of S. gaudichaudii is rounded, nearly as broad as long ; it bears three dorsal carinae, 

 a median one interrupted in the middle, and two lateral ones, which are curved and 

 follow the outline of the segment as far as the bases of the uropoda where each terminates 

 in a very small spiniform process : these carinae are less pronounced in this species. In 

 S. laevis the median and lateral carinae of the terminal segment are almost obsolete. 



(4) The colour of the three species differs : that of S. convexa is a uniform pale brown, 

 that of S. gaudichaudii is brown, darker towards the middle line and dotted all over with 

 black spots, whilst that of S. laevis is white. 



(5) The three species resemble one another and differ from the majority of species in 

 possessing maxillae in which the two articulating lobes are broad ; and each lobe bears 

 eight or nine pectinate setae instead of the more usual number of two. The distal joint 

 of the palp of the maxilliped is also smaller than usual. 



(6) The two species S. convexa and S. gaudichaudii, and probably also S. laevis, are 

 characterized by a sexual difference not found in any other species ; the form of the 

 modified setae which occur on the propodus of the second thoracic appendage differs in 

 the two sexes (see descriptions of species). 



(7) Long plumose setae are present on the carpus of the second thoracic appendage, 

 and on the posterior margins of the propodus, carpus, merus and ischium of the last 

 thoracic appendage of the adult male in all three species. 



(8) A further sexual character of the adult male of S. convexa and S. laevis is the 

 presence of a patch of plumose setae on the sternum of each of the third to the seventh 

 thoracic somites. I have not been able to examine adult male specimens of S. gaudi- 

 chaudii, and I am unable to say whether or not it also possesses this character. 



