338 STEPHEN G. RICH 



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turn are exactly as in all the forms hitherto described, but the shift 

 of the gills brings each longitudinal band in the middle of a 

 double row. 



Internally, the rectum bears six double rows of gills, three on 

 the right side and three on the left. The gills themselves are 

 elongate protuberances, flat, with rounded ends, and undercut 

 on one edge, as shown in figure 26. The arrangement of cush- 

 ions and fat differs essentially from that in the Aeschnidae. 

 There is but one cushion, which is on the cephalic side of the 

 gill. The cushion is ovate in form, and overlies the mass of fat, 

 which fonns a triangle at the base of the caudal edge of the gill. 

 Both cushions and fat are less prominent than in any aeschnid 

 form. The tracheation of the p"ill does not in any way differ- 

 from that of the aeschnid buttress fold except that the tracheae 

 do not pass through the fat in the base of each gill, but cephalad 

 of it. 



The gills are not set at the same angle as their bases. They 

 stand about 15° from the sagittal line, and those of each iDw over- 

 lap like tiles. There are twenty-eight gills in most of the single 

 rows. Figure 27 shows the arrangement of the gills. 



On each gill three buttons, pads, or tubercles are found near 

 the distal rounded margin. The most distal one is on the caudal 

 side, the other two on the cephalic side of the gills. Between 

 these and the thicker parts of the gill at the base, the water has 

 space to circulate around the parts of the gill filled with tracheal 

 loops and fine tracheoles (fig. 26). 



The cross-section of the rectum of the allied genus Libellula 

 (fig. 28, from Miss Andrews' MS.), shows some of the relations 

 of the parts quite clearly, and indicates well the differences from 

 aeschnids. The gills are of the same size in all parts of the 

 rectum; in only a few specimens those in the cental part were a 

 little larger than the others. There is no anal canal, the rectum 

 tapering quickly and directly to the anal valve. 



