﻿26 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



and 28). Its proportions do not vary much individually, at least 

 not in the species of Lepidurus, and are well worth using as a tax- 

 onomic character, though they do not always separate species as well 

 as in the case just mentioned. 



The presence of a supra-anal plate has for a long time been the mam 

 character of the genus Lepidurus. Sometimes we meet forms of Apus 

 (see fig. 8, and also Gautliier, 1934, pi. 2, fig. D) which show the appear- 

 ance of such a plate not much smaller than that in some specimens of 

 L. arcticus. However, for practical purposes the character seems to be 

 quite acceptable. 



The supra-anal plate has always been regarded as a useful taxonomic 

 feature, and this course seems to be justified, provided we do not pay 

 too much attention to its form. Holmes (1894, p. 586) has an ex- 

 treme example of variation in this respect in a lot of only 5 specimens 

 of Lepidurus lemmoni, in which it "may be rounded, truncated 

 (obliquely so in some specimens), or, in some cases, bilobed." Bilo- 

 bation of this plate is not a valid character. Sars (1896) figures 

 such a condition in some specimens of L. arcticus, and Spandl (1925) 

 the beginning of a bilobation in the European species L. apus. In the 

 new species of Lepidurus from the State of Washington, I have 

 observed varying bilobation, or none, in specimens of both sexes and 

 of all sizes (figs. 15 and 24). Having found in several cases a varia- 

 tion in this respect within the limits of a species, I cannot, of course, 

 follow Sidorov (1927) when he uses the bilobation as a generic 

 character. 



The largest supra-anal plate among all Notostraca is found in 

 males of L. couesii, where its length can approach 44 percent of the 

 length of the carapace (fig. 11). In L. arcticus, on the other hand, it is 

 unusually small, being 7 to 13 percent of the length of the carapace. 



The spines on the dorsal side of the supra-anal plate are sometimes 

 valid characters, though their number is not fLxed for each species but 

 varies vdthin certain limits. I have noticed 0-4 of them in L. arcticus, 

 4-19 in L. packardi, and 2-7 in the new species oi Lepidurus. In these 

 species they always occur m a reasonably straight row, but not on a 

 keel, and they are rather similar in size. The two remaining North 

 American species of Lepidurus are very different in this respect. In 

 L. couesii there is a strong middorsal keel covered with very numerous 

 spines (20-100) that vary considerably in size (figs. 9 and 11), most 

 of which are very small and in this respect are similar to what I have 

 called accessory small spines sometimes occurring on the ventral 

 side of the legless body-rings in several species (p. 19). A varying 

 number reach a size comparable with that found in other species of 

 the genus, though not quite so large, and there are intermediate sizes, 

 too. L. bilobaius, of which I have seen only a few specimens, has a 

 supra-anal plate similar to that of L. couesii, though the keel is less 



