﻿4 PROCEEDINGS OF THE NATIONAL IMUSEUM vol. 102 



populations, although the real status of these cannot be determined 

 from the accessible material. In connection with this analysis I 

 have also considered polypody, and the peculiar phenomenon I have 

 called spiral growth, both of which are problems of interest from a 

 more strictly morphological point of view. Finally, I have applied 

 to the North American material the results of this analysis of taxo- 

 nomic characters. 



TERMINOLOGY AND MEASUREMENTS 



Instead of the term "segment," in referring to the postmaxillary part 

 of the animal, I prefer the term ''body-ring." The first 11 body-rings 



I call the thorax. This is followed by the abdomen, which is com- 

 prised of leg-bearing body-rings, legless body-rings, and the telson, 

 which is not a true segment, and which should not be included in the 

 number of body-rings. The series of legs does not always stop under- 

 neath a boundary between two body-rings; it may stop at any place 

 quite independent of these boundaries (fig. 20). I have found it 

 advisable to count half leg-bearing body-rings in those cases where 

 the series stops approximately underneath the center of a ring, and a 

 whole leg-bearmg ring when the series covers almost the whole ring, 

 disregarding those cases where the series only slightly surpasses a 

 boundary. Incomplete rings (fig. 16) are often found at the anterior 

 boundary of the telson, but very rarely within the series of rings. 



It is useful to have a short formula when describing the numbers 

 of the various kinds of body-rings in a specimen or in a species. In 

 this paper, I have used such an arrangement. Thus, the formula 



II + 12 + 5=28 body-rings indicates that there are 11 thoracic, 12 

 abdominal leg-bearing, and 5 abdominal legless rings, forming a total 

 number of 28 rings (telson not included). For a species the formula 

 is more complicated because there is a considerable variation of 

 these numbers within the limits of a species (Linder, 1947). If the 

 abdominal leg-bearing rings number from 16 to 18.5, the legless rings 

 from 5 to 10, and the total from 35 to 38 rings, the formula would be, 

 11 + (16-18.5) + (5-10) =35-38 body-rings. If there is an incomplete 

 ring, the abbreviation "i" is inserted, as in this example: 11 + 12 + 

 5+i=28+i body-rmgs. It is also possible to indicate the presence 

 of the interesting abnormality, spiral growth, and its place within 

 the series. If a spiral of 3 rounds appears after the 25th body-ring, 

 immediately followed by the telson, in a specimen with 12 abdominal 

 leg-bearing rings, the formula would run as follows; 11+12+ 

 2+sp.3r. = 25 + sp.3r. ' To 



The telson bears a rather neglected structure, viz, a pair of dorsal 

 sensory setae (fig. 28), surrounded by short spines, and a pair of caudal 

 filaments. Betw^een the latter there is, in the genus Lepidurus, the 



