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



Ectinosoma atlanticum (?) (Brady and Robertson), (PI. IV. figs. 10-14). 



Microsetella atlantica, B. and B., Aim. and Mag. Nat. Hist., ser. 4, vol. xii. p. 130, pi. ix. 



figs. 11-1G (1873). 

 Ectinosoma atlanticum, Brady, Monograph of the British Copepoda, vol. ii. p. 13, pi. xxxviii. 



figs. 11-19. 



Length, l-45th of an inch ('53 mm.). Body very slender, almost linear, much 

 attenuated both behind and in front ; tail-segments very short and divergent. First 

 four pairs of feet long and slender (fig. 12), outer branches slightly shorter than the 

 inner, each joint bearing a long, slender, apical spine, and pectinated on its outer edge ; 

 median joint only of the inner branch pectinated. Fifth pair of feet (fig. 13) rudi- 

 mentary, two-jointed ; internal portion of the basal joint produced nearly as far as the 

 apex of the second joint, and bearing two apical setas, one of which is very long ; second 

 joint smaller, and bearing two long and one or two very short setse. Caudal segments 

 bearing two principal setse, one of which is longer than the body of the animal, the other 

 about half as long. The posterior borders of the last three abdominal segments are finely 

 and densely pectinated (fig. 14). 



The foregoing imperfect description gives, with as much accuracy as could be obtained 

 from observation of only a single specimen, the characters of the species. Though 

 differing very slightly from those of Ectinosoma atlanticum, I cannot venture on such 

 slender grounds to assign to this single example a new specific name. It was taken in 

 the tow-net down to 200 fathoms, on the 9th of April 1876, in lat. 3° 10' N., long. 14° 

 51' W. (near Ascension Island). 



Dr. Giesbrecht has recently 1 proposed to split up the genus Ectinosoma, leaving in it, 

 if I rightly understand him, only one species, Ectinosoma gothiceps, Giesbrecht. But 

 the' grounds for this proposal are as yet given only in a very cursory way, and, so far as 

 I can judge, are insufficient. Should the present species eventually be assigned to a 

 different genus, the name Microsetella (withdrawn in the Monograph of the British 

 Copepoda) must be reinstated. 



Pseudothalestris, n. gen. 



Like Tlialestris, except as to the structure of the first pair of feet, in which the outer 

 branch is very short and only two-jointed, the inner branch long, three-jointed, having 

 the first joint very long, the second and third rudimentary. 



1 VorlSufige Mittheilung aus einer Arbeit iiber die frei lebenden Copepoden des Kieler Hafens Zool. of Anzeiger, 

 No. 83, 1881. 



