﻿NORTH AMERICAN NOTOSTRACA — LINDER 9 



of North American Notostraca. There are few exceptions to this rule. 

 Packard (1883) mentions the figures for one specimen of Apus lucasa- 

 nus and Rosenberg (1947) quotes fixed figures characteristic of his 

 species A. oryzapkagus and A. biggsi. The number of legless rings, 

 on the other hand, is almost always recorded, and a slight variation 

 in this respect is often noticed. Interesting information about 

 Lepidurus arcticus is given, though only with some hesitation, by 

 Chamberlin and Duncan (1924, p. 99): "The number of segments 

 visible on the dorsal surface from the point of attachment of the 

 carapace to the body seems to be either 26 or 27. . . . Behind the 

 appendage bearing segments are five others, exclusive of the telson, 

 which are without appendages of any sort." As the above-mentioned 

 point of attachment stretches over the first body-ring, we may con- 

 clude that these authors found the number of body-rings to be 

 11 + (11-12) + 5=27-28 (telson not counted). 



Even in descriptions of species from other parts of the world we 

 seldom get information about the number of leg-bearmg abdommal 

 rings; at times a fixed total number is given, together with the number 

 of legless rings, but without any attention to a possible variation of 

 the former figure. Zaddach (1841) provides an exception; he says 

 that in A. cancriformis the number of leg-bearing abdominal rings 

 (in "pars abdominis posterior," according to Zaddach) is 17 or 18, 

 the number of legless rings 5 or 6, and the total number of body rings 

 always 34. 



All authors except Chamberlin and Duncan seem to take it for 

 granted that the species of Notostraca are nomomeristic, but this is not 

 true of an)^ of the species examined by me. 



From figures 14 and 31 it can be seen that a variation in number is 

 found not only in the legless rings, but in the leg-bearing ones, too. 

 These varying numbers, together v/ith the 11 thoracic rings, are 

 combined in different ways to give a total number varymg within the 

 limits of a species. This is true of every species of which I have seen 

 a reasonable number of specimens. 



When I now proceed to analyze what I have found concerning 

 the variation of these figures — a preliminary analysis in some respects 

 because the material is rather scanty — I shall begin, not in the tradi- 

 tional way, with a treatment of the legless rings, but with the total 

 number of rings and the number of abdominal leg-bearing rings. 

 These are the primary characters. The number of legless rings is not 

 a simple character; it is the result of the varying interplay of the two 

 other series. 



As for the total number of body-rings, the range of variation may be 

 quite considerable in specimens from the same locality. In a new 

 species oi Lepidurus from Grand Coulee, Wash, (see p 39), the males 



