COPEPOD GENUS HEMICYCLOPS — GOODING 163 



were viewed simultaneously with the images of later mounts similarly 

 oriented through the appropriate combination of lenses. Scales of 

 magnification for each of the lens combinations used were drawn from 

 a stage micrometer. The method for stereoscopic microscopy de- 

 scribed by Galbraith (1957) proved very helpful in determining the 

 shape and relations of appendages. 



Among the many persons and agencies who have assisted me from 

 time to time, I wish to thank particularly Dr. Paul Illg for much 

 helpful discussion, suggestions, and encouragement during all phases 

 of this work; Drs. Ernst Florey and K. L. Osterud for their critical 

 reading of the manuscript; Drs. Fenner A. Chace, Jr., and Thomas 

 E. Bowman of the U.S. National Museum for arranging the loan of 

 material from the National Museum collections; and several others 

 named in the text for supplying me with specimens either of copepods 

 or of their hosts and information. The State of Washington, through 

 Dr. Illg, provided support for part of the woik and some facilities. 



Genus Hemicyclops Boeck 



Hemicyclops Boeck, 1873, p. 42; etc. 



Of the seven species of Hemicyclops previously described from North 

 and South America, three are considered valid and one doubtful; 

 two new ones are added. Although very few specimens of each 

 species have been examined, the small amount of variation between 

 individuals of a particular form has made it possible to be some- 

 what more definite on points of specific discrimination than the evi- 

 dence available might seem to favor. The following morphological 

 characters are considered to have taxonomic value in differentiating 

 the species discussed: the nature of the genital segment in the adult 

 female; the ornamentation of the antenna and its elements; the ter- 

 minal armature of the mandible, maxilla and maxilliped; the orna- 

 mentation of the labium; the setal formula of the swimming legs; 

 the relative lengths of elements on the fifth legs; the presence (and 

 nature) or absence of sixth legs; and the modification of setae on 

 the caudal rami. These characters permit us to divide the American 

 species into two well-defined groups: the first comprising thysanotus 

 (and purpureas if it occurs in these waters, see p. 176), the other includ- 

 ing adhaerens, subadkaerens, arenicolae, and probablv elonrjatns. (Lack 

 of knowledge about the mouthparts and maturity of the only specimen 

 available precludes any completely certain disposition of elongatus.) 



It is difficult at this stage to determine whether the second group 

 represents a polytypic species or a superspecific complex. I have 

 preferred the latter alternative because the morphological differences 

 between its members are approximately equivalent to those currently 



