COPEPOD GENUS RIDGEWAYIA — WILSON 163 



The specific difference noted in the two Tortugas species in the 

 jointed basal portions of the two middle caudal setae is a valuable 

 character for distmguishing whole specimens, and particularly the 

 females of associated species. It is not mentioned for any of the 

 other species, but should be recorded for all species encountered in 

 future studies. 



In all of the species the rostrum appeal's to be a large, somewhat 

 expanded structure, without filaments. 



Antennule: The antennules of the two new species have been 

 studied in detail at high magnification with oil immersion objectives. 

 The material has been critically checked and rechecked, partly because 

 the antennules of the female and that of the left side of the male are 

 26-segmented, differing thus from the segmentation recorded for 

 typica and marki, or recognized for any other calanoid species. The 

 greatest number of segments that has been conceded to be present 

 in the antennule of existing calanoids is 25. The few instances in 

 which a 26-segmented antennule has been reported are thought to be 

 due to the fact that the observer included the surface eminence to 

 which the antennule is attached. Whole specimens of both species 

 as well as dissected antennules have been examined with this in mind, 

 so that such an error would not be repeated in the case of these two 

 species of Ridgeivayia. 



Gurney (1931, pp. 40-48; 1933, pp. 46-61) has discussed the inter- 

 pretation of the armature and development of the antennule in rela- 

 tion to its evolution, and points out (1931, p. 42) that the primitive 

 antennule of calanoids probably consisted of 27-28 segments, or even 

 of 30 or 31 segments. It seems apparent in all species that some of 

 the fusion leading to reduction has taken place in the proximal part 

 of the antennule, particularly in the usual second segment of a 25- 

 segmented appendage. This segment is usually comparatively long, 

 and bears more than the two setae and aesthete considered to repre- 

 sent the archetypical grouping for each segment. The second segment 

 shown in the illustration of the antennule of typica (Thompson and A. 

 Scott, 1903, pi. 1, fig. 3) is elongate and appears to have two or even 

 three groups of setae. On the basis of length it is comparable to 

 segments 2 and 3 of gracilis and shoemakeri, indicating that the differ- 

 ence in segmentation of these congeners may be due to fusion in these 

 proximal segments. Reference to the summary of setation given 

 herein in the description of gracilis, and found to be identical in 

 shoemakeri, shows that what is considered a primitive armature, as 

 well as segmentation, has been largely retained — most segments have 

 two setae, and aesthetes are abundantly distributed. 



