﻿NORTH AMERICAN NOTOSTRACA — LINDER 27 



well marked and the spines are fewer, 4-6 large ones and a trifling 

 number of small ones (see fig, 13). 



In L. couesii, the number of spines on the supra-anal plate obviously 

 increases with the size of the animal, but in other species I cannot say- 

 that tliis is the rule. 



The spines around the dorsal sensory setae and around the bases of 

 the caudal filaments usually do not present valid characters in Ameri- 

 can forms, and as for the conspicuous spines placed more middorsally 

 on the telson in the genus Apus, they vary in number in almost every 

 sample. Later on (p. 59), I will give examples of this that may go 

 to show that the number is not a reliable character. This variation 

 also occurs in several extra-American species of Apus such as A. 

 numidicus (Barnard, 1929, p. 234), A. granarius, and A. australiensis. 

 The caudal filaments present good sexual characters in the armature 

 of their basal parts (Gurney, 1924, 1925), because those of the female 

 are bristly and those of the male armed, at least at the base, with 

 roimded knobs. This, however, seems to apply to forms of Apus 

 only. In other respects I have not found these filaments useful for 



taxonomic purposes. 



THE CARAPACE 



The carapace is more flattened in the male than in the female. 

 The relation of its width to its length is, according to Barnard (1929), 

 a valid character in some South African species of Apus (though not 

 m all of them!). I have given this relation in the descriptions, but, 

 at least in Apus, I have found no conspicuous differences in the Ameri- 

 can material. Many spines, which sometimes are very small, often 

 occur in both genera on the dorsal surface, especially on the carina. 

 The presence, absence, or number of these structures cannot, as it 

 would seem, be used as a taxonomic character ; many times I have 

 found in a single lot great variation in this respect, without any 

 correlation with other characters. Barnard (1929) found similar 

 variation in South African material of Apus and he, too, rejects its 

 taxonomic importance. The lateral margin of the carapace is regu- 

 larly furnished with some very small spines, at least in its posterior 

 part, and in the variety of the new species of Lepidurus described here, 

 these spines are large and conspicuous around most of the margin 

 (pi. 5, fig. 1). As for the posterior emargination of the carapace, 

 early authors used to pay a great deal of attention to the number of 

 spines found on it, and this character stiU appears in descriptions, 

 sometimes with pretensions of separating species. However, Braem 

 (1893) and many authors after him have found a great range of 

 variation in this respect in many species, and so have I in the American 

 material. On the other hand, the size and pattern of these spines in 

 several cases may prove helpful to the taxonomist. In most forms 

 the spines are fairly equal in size and regularly placed, but sometimes 



