COPEPOD GENUS RIDGEWAYIA — WILSON 167 



modified antennule. If this is a true correlation, then the elongate 

 segment preceding the genicidation represents a fusion of three seg- 

 ments. Such an interpretation is easily supported in shoemakeri 

 (fig. 29), which has three groups of spinides and three setal groups; 

 in gracilis, the comb of spinules obscures any middle setal group. 



These two Tortugas species are thus not only unusual among Cal- 

 anoida in the segmentation of the antennule but also in the position 

 of the specialized hmge. It is not too surprising to find calanoid cope- 

 pods with 26-segmented antennules, but the difference in the hmge 

 position is unexpected. This seeming departm-e from what has been 

 considered a basic pattern raises the question as to how well the facts 

 are known. In studying the literature it is apparent that knowledge 

 of detail of antennule structure and armature is lacking for many 

 species and genera. Most of the available data of worth come from 

 the observations of early workers (Claus, Schmeil, Giesbrecht) ; among 

 the most miportant examples are the incomparable, detailed figures 

 of Giesbrecht (1892). Suice then few workers have given more than 

 the rudiments of antennule structure in their descriptions. This is 

 unfortunate since it is apparent that some very exact patterns of 

 segmentation and armatm-e have been established in the evolu- 

 tionary development of this appendage in the Calanoida, and it 

 therefore has high significance at all taxonomic levels. 



Antenna: This appendage is figured for all the species except 

 marki, for which Esterly (1911) describes the exopod as 8-segmented. 

 This agrees with the other species except canalis, which Gurney 

 (1927) shows as havhig a 7-segmented exopod. All segments have an 

 inner, lateral seta except the last, which has four apical setae in all 

 except typica, for which three are showm (not known for marki). 



Oral appendages: Where known, the mandible and maxilliped 

 show no significant differences. The maxillule is unloiown for canalis, 

 but agrees closely in the other species. A single epipodal seta is 

 present in gracilis and shoemakeri but is not shown for typica and 

 marki, though the lobe is present in the illustration of the latter. 

 This point should be checked in future studies of these two species 

 as it m.ay be of taxonomic importance. The number of setae shown 

 in the figures of typica and marki on the various lobes and laciniae, 

 and on the exopod and endopod, show slight differences from one an- 

 other and from the new species. As some of these may have specific 

 value and should be chocked in future studies, they are summarized 

 here (table 1). 



The maxilla appears to have the most taxonomic importance of all 

 the oral appendages in species differentiation, inasmuch as the nmnber 

 of lobes varies from five in canalis to seven in typica. The reduction 



