166 PROCEEDINGS OF THE NATIONAL MUSEUM vol.108 



preceding these has the appearance of at least two coalesced segments 

 comparable to the two distinct segments of shoemakeri. The two 

 groups of setae on the fused segment of gracilis correspond to those of 

 the two separated segments of shoemakeri not only in placement but 

 also in modification. It therefore appears that the geniculation occurs 

 at exactly the same point, although the number of free segments 

 beyond the joint is different. 



In both gracilis and shoemakeri there are 20 free segments preceding 

 the geniculation. There would appear to be no question that the 

 modified segments of the midportion counted as segments 13 and 14 

 represent two segments (fig. 30). It therefore seems impossible to fix 

 the position of the hinge at a point comparable to that presumed to be 

 identical for all calanoids. It has long been held, as Gurney (1931, p. 

 47) emphasizes, that the ". . . position of the hinge may be regarded 

 as a fixed point. In the Calanoida, this point is always between segs. 

 18 and 19." In these two species of Ridgewayia it seems correct to 

 assume that the 26-segmented unmodified antennule of the male and 

 female has resulted from a lack of fusion of two segments of the proxi- 

 mal area and may differ from the closely allied species typica in the 

 division of these segments. The modified antennule agrees with the 

 unmodified antennule in relative length and armature of the proximal 

 segments. In comparing this 26-segmented antennule to the basic 

 calanoid 25-segmented appendage from whose study the hinge position 

 has been derived, it would seem necessary only to add one segment to 

 arrive at a comparable position. The hinge in these two species of 

 Ridgewayia should therefore fall between segments 19 and 20, but this 

 is not the case. Not only are there 20 countable segments preceding 

 the geniculation but the elongation and armature of the 20th segment 

 suggests that it may have resulted from fusion of at least two or even 

 three of the segments of the preceding stage. 



Comparison of the armature of the unmodified antennule with that 

 of the modified, may not indicate what segments are mcluded in the 

 fused distal region of the right appendage. But attention should be 

 drawn to similarities that characterize certain areas of each in these 

 two species. The proximal and the two distal segments are alike in 

 armature and length. The groups of surface spinules occur on identical 

 segments of the left and right side in both species (13-22 and 24-25). 

 Segment 22 has a particularly long seta on the unmodified antennule; 

 such a long seta is present on the segment preceding the geniculation 

 in both species. Jointed setae are present on the unmodified anten- 

 nule beginning with segment 23 ; similar setae are present beyond the 

 geniculation point of the right antennule. Thus, on the basis of com- 

 parison of armature, the distal part of the segment preceding genic- 

 ulation (visible segment 20) is comparable to segment 22 of the un- 



