﻿28 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



(in L. packardi and L. bilobatus) there are large spines interspersed 

 between much smaller ones at fairly regular intervals, and this seems 

 to be worth considering in the taxonomy, because the individual 

 variation in this respect is very slight in the material I have examined. 

 The depth of the posterior margination is also worth attention. 



On the ventral side, just in front of the antennae, we find the 

 supra-antennal crest. Simon (1886) attaches some importance to 

 the appearance of this crest, which in some forms is quite smooth 

 and in others is furnished with small tubercles or is almost serrated. 

 As a taxonomic character, this seems to be well worth attention. 



THE NUCHAL ORGAN 



Barnard (1929) found that the height and location of the tubercle 

 on which the nuchal organ is placed were very useful in distinguishing 

 the South African species of Apus. The North American forms of 

 the genus, however, are uniform in this respect. In Lepidurus, I have 

 not observed any considerable differences in the height of it, and in 

 almost all species its location is more or less uniform, that is, it occurs 

 somewhat between the hind part of the eye-tubercles (fig. 19 and pi. 7, 

 figs. 3-6). Aberrant in this respect is the new species from Washing- 

 ton, in wliich it occurs considerably behind the eye tubercles. The 

 fact that the eyes are unusually small in this species may have some 

 bearing on this, however. The same conditions occur in the variety 

 of this species. According to Holmes (1894), the nuchal organ in L. 

 lemmoni is "located considerably behind the posterior margin of the 

 eyes." As the posterior margin of the eyes is situated in front of 

 the posterior ma^rgin of the eye-tubercles, we cannot say for certain 

 whether the organ is placed behind the latter margin or not. 

 THE SECOND MAXILLAE 



Sars (1901) observed that the second maxillae are relatively larger 

 in Lepidurus than in Apus, though ''the exopodal part" may be much 

 larger in Apus. ^Yhat he calls "the exopodal part" is only the free 

 end of the efferent duct of the maxillaiy gland. Tliis is called "palpus" 

 by Wolf (1911, p. 260), who made another important observation: 

 "Vielleiolit diirfte auch das von mir an vielen Arten beobachtete 

 Unterscheidungsmerkmal allgemeine Geltung haben, dass namlich 

 bei Lepidurus die 2. Maxille seitlich einen Palpus tragt, wahrend 

 bei den Triops-Arten dieselbe voUstandig rudimentar geworden, und 

 der bedeutend vergrosserte Palpus frei zu stehen scheint." A similar 

 reduction is mentioned by me in an earlier paper (1945, p. 13). I 

 can now confirm that in most species of Apus {A. australiensis, 

 A. granarius, A. numidicus, and A. longicaudatus) middle-sized and 

 large specimens have only the free end of the efferent duct of the 

 maxillary gland remaining. In small specimens of these species the 

 reduction has not gone so far as that, the maxillae being similar to 



