﻿18 PROCEEDINGS OF TPIE NATIONAL MUSEUM vol. 102 



In samples of unusually well-preserved specimens I have observed 

 a great range of variation in the number of exposed rings, particularly 

 in L. couesii (2-8), in the new species of Lcpidurus (10-19), and in 

 A. longicaudatus (20-26). No correlation with other characters is 

 noticed in these samples, with the exception that specimens with 

 many exposed rings in most cases have longer rings than those with fewer 

 exposed but with a carapace of about the same size. The appearance 

 of the rings supports the idea that the variations are due to contraction. 



Nevertheless, we must not altogether overlook this character. By 

 their large number of exposed rings, Lepidurus bilobatus and the new 

 species of Lepidurus (pis. 2, 3, and 4) are distinguished from the other 

 species of the genus (though L. arcticus, too, sometimes may have a 

 rather large number, as may be seen on plate 1), and suggest the genus 

 Apus. It is quite a conspicuous character, and one that is easily 

 explained by the great number of abdominal rings in these species. 

 However, the variety of the new species described in this paper has 

 just as many abdominal rings as do these forms, but only 7 to 10 

 exposed rings as against 10 to 19. Here the posterior emargination of 

 the carapace is not nearly so deep as in the typical form, and this is 

 certainly part of the explanation. However, the difference in number 

 seems a little too great to be explained by this fact alone. One is 

 tempted to say that the carapace is, relatively, a little larger here 

 than in the typical form. Measiu-ements confirm this; not only is 

 it larger in relation to the total length but also in relation to length 

 or width of separate body-rings. The body-rings are set just as 

 closely together in the principal form as in the variety, and thus there 

 appears to be a real difference in the relation between the length of 

 carapace and of body in the two forms. 



We see from the above that the niunber of exposed rings is not a 

 simple character, but is the result of the combined action of several 

 separate features, one of which is subject to variation caused by 

 artificial means. Considerable caution is necessary when dealing 

 with this as a taxonomic character. 



The same thing may be said about the length of the carapace in 

 relation to the length of the exposed part of the body. Packard 

 (1883), when separatmg his species within the genus Apus, makes 

 extensive use of this character. Because of the considerations noted, 

 I do not think it advisable to pay much attention to this feature. 



SIZE AND ARMATURE OF THE BODY-RINGS 



As we have seen, the lengths of the individual body-rings, though 

 not used as a separate character, influence two that are commonly 

 used: the number of exposed body-rings, and the length of the cara- 

 pace in relation to the total body length. It should be made clear, 



