50 THE VOYAGE OF H.M.S. CHALLENGER. 



females differ from each other in their shape ; as in Serolis schythei, the male is much 

 broader in proportion to its length, being nearly circular in outline ; in the female 

 the shape of the body is oval ; this difference depends upon the greater length of the 

 epimera ; in the male the sixth pair extend nearly so far as the end of the terminal 

 spine of the caudal shield, while the two abdominal epimera extend about half way 

 down the caudal shield, their posterior extremities being situated on a line with the 

 articulation of the uropoda ; in the female the sixth thoracic epimera only just pass 

 beyond the articulation of the uropoda, while the abdominal epimera hardly reach as far. 



The sterna of the abdominal segments differ in the two sexes ; in the male the two first 

 are oblong in shape, the posterior margin being slightly concave, the third is prolonged 

 into a fiat triangular spine ; in the female the posterior margin of all the segments is 

 prolonged into a blunt spine ; tlie length of these increases progressively from before 

 liackwards. 



Studer has given a good general description of this species, comparing it with Se7'oiis 

 trilohitoides, to which it is most nearly allied. For the sake of completeness I give here a 

 recapitulation of his description, with a few additions, which applies to all the specimens 

 in the Challenger collection, the variations being inconspicuous and confined to the colour, 

 which is darker or lighter, and the number of spines upon the carina of the caudal shield, 

 which vary from three to six. 



The length of the largest male specimen is 34 mm., its breadth 31 mm. ; the length of 

 the largest female specimen is 41 mm., its breadth 35'5 mm. 



The length of the cephalic shield is about one-fifth of the whole length of the body ; 

 close to the anterior margin is a transverse ridge which runs almost parallel to it ; in 

 front of the eyes, and occupying the middle of the cephalic shield, is a squarish area 

 raised and separated off by a groove ; the posterior margin of this is slightly concave, 

 the two ends being somewhat produced backwards ; the lateral portions of the cephalic 

 shield in front of the eyes slope gradually downwards towards the side ; behind and to the 

 inside of the eyes is a large blunt conical tubule, one on either side ; between the two, and 

 forming the posterior boundary of the cephalic shield, is a flattened triangular area. 



Thorax. — The epimera are broad and sickle-shaped, and the terminal portions from 

 the articular process outwards project freely and do not overlap each other ; the 

 articular processes are short, and as in many other species, there are only two to 

 each epimeron ; the posterior articular process of each epimeron overlaps the anterior 

 articular process of the succeeding epimeron. 



The second, third, and fourth epimera are separated from their terga by a distinct 

 suture. The anterior epimera are separated into two halves by a transverse ridge, and 

 there is in addition an anterior ridge as in Serolis hromleyana and Serolis gracilis, which 

 is a continuation of the anterior ridge on the cephalic shield ; it runs at first parallel to 

 the anterior margin of the epimeron at some little distance from it, and then bends 



