268 PROCEEDINGS OF THE NATIONAL MUSEUM vol. u7 



Remarks. — This species has been misidentified by Wheeler, 

 Wilson, and probably other authors. Wheeler's illustrations, re- 

 printed in Wilson (1932b), agree closely with M. Dahl's figures of F. 

 gracilis, said by Dahl to be an Atlantic species. F. carinata has a 

 rather different urosome shape (see Dahl, 1912, pi. 15, fig. 22) and is 

 considered by Dahl to be a Pacific species. 



The urosome of F. concinna (Dana) is similar in shape to that of 

 F. gracilis but is lower and longer. Although M. Dahl states that it 

 is a Pacific species, Moore (1949), Bjornberg (1963), and Owre (1962) 

 have reported it from the Atlantic. 



Family Oithonidae 



Prosome and urosome quite distinct; prosome moderately wide; 

 urosome narrow. Female Al rather long, armed with long setae, 

 without aesthetascs; both male Al geniculate, terminal segment with 

 aesthetasc. A2 2-4-segmented; Re absent. Mouthparts biting. Re 

 and Ri of P1-P4 2- or 3-segmented. P5 a small conical segment 

 (sometimes lacking) with 1-2 apical setae; another seta on side of 

 segment anterior to P5. 



Genus Oithona Baird 



Head rounded or produced into beak-shaped rostrum. A2 2-3- 

 segmented. Md with small 1-segmented Ri and 4-segmented Re; B2 

 with 2 setae on medial margin. Re and Ri of Mxl 1-segmented. Re 

 and Ri of P1-P4 3-segmented except Ri of Pi, rarely 2-segmented. 



Members of the genus Oithona are among the most numerous and 

 characteristic plankters of inshore waters throughout the world. The 

 genus is large and the characters used to separate some of the species 

 are shght. Some species have been described inadequately; e.g., such 

 important characters as P5 and the terminal setae of the Md have 

 been omitted in some descriptions. Identification consequently is 

 sometimes difficult, and we therefore give rather detailed diagnoses of 

 the Puerto Rican species and the artificial key below. 



Kiefer (1935) removed the species of Oithona having 2 terminal 

 setae on P5 and estabhshed the genus Dioithona for them. His 

 decision, based on long experience with freshwater cyclopoids where 

 the armature of the rudimentary P5 has high taxonomic value, has 

 not been generally followed. We are not convinced that the genus 

 should be divided on the basis of a single character; a thorough 

 revision, using all available characters, is needed to determine the 

 vaUdity of Kiefer's action. Of the Puerto Rican species, only Oithona 

 oculata is a Dioithona. 



