﻿NORTH AMERICAN NOTOSTRACA — LINDER 21 



preceding the telson occurs fairly often. This incomplete segment is 

 visible on one or the other side, or only on the dorsal surface." He 

 does not count it when giving the niunber of rings. Incomplete body- 

 rings occur frcquentlj'"; in one lot of the new species of Lepidurus 

 described herein, for example, 10 out of 54 males and 7 out of 42 

 females show this structure in varying degrees of development. 

 Nearly half the specimens in a lot of A. longicaudatus have incomplete 

 rings: 14 out of 33 males, and 44 out of 98 females (fig. 31). These 

 examples show that it is best not to omit them when counting the 

 rings. They occur at any place around the front boundary of the 

 telson (I have found, in two cases only, a piece of an incomplete ring 

 in the middle of the series of abdominal rings) and may be of any 

 size from a tiny piece to an almost but not quite complete ring. Two or 

 even three of them may occur in the same specimen (fig. 16). Under 

 their integument there is always a continuation of the longitudinal 



3^th J" ■/ 



body-rfng // 1/ , „ >a-\/— ^ 



Figure 6. — a, Ventral view of portions of the abdomen and telson of a female of Apus 

 longicaudatus LeConte (U.S.N.M. No. 18908) showing a right-handed spiral of a little 

 more than one and one-half rounds, beginning in the midventral line, and preceded by a 

 one-round structure similar to a spiral, X 29. {T, telson.) h, Schematic drawing. 



muscles. I cannot say, as yet, whether such a piece sometimes grows 

 to be a complete ring. The existence of a piece of an incomplete 

 ring within the series of abdominal rings shows that they do not neces- 

 sarily become complete rings and that they may be followed by a 

 normal ring caudaUy. 



SPIRAL GROWTH 



Occasionally the regular growth is disturbed, and the rings in some 

 part of the body are replaced by a real spiral (Linder, 1947). In more 

 than 2,000 North American specimens I have foimd 15 cases of this, 

 some of which are illustrated in figures 3 through 7, 17, and plate 7, 



