326 STEPHEN G. RICH 



end of each but'tress fold at the point where it joins the main 

 fold, and the buttress fold ends in a sharp angle here. In 

 figure 3 the longitudinal folds are shown in section. 



I was unable to see the musculature of the rectum of this 

 species, but saw it in Hagenius brevistylis, where there are six 

 compact longitudinal bands of muscle, three on the right and three 

 on the left of the middle line of the rectum. They alternate in 

 position with the double rows of gills. The circular muscles, 

 which are outside the longitudinal, are simply a loose series of 

 rings of muscle fiber around the rectum. 



FEATURES COMMON TO ALL FORMS 



Before passing to a survey of the Anisoptera by genera, it 

 may be well to summarize the points in which the description of 

 the rectum of Cordulegaster, just given, is a description of all 

 the recta studied. The following features are common to all 

 these forms : 



1. The distribution of tracheal trunks is the same in all 

 genera. The only differences are in the places and manner in 

 which the trunks become larger or smaller and in the number 

 and grouping of their branches. 



2. In the regions in which the branches to the rectum leave 

 the trunks, no branches to other organs originate. From each 

 postdorsal trachea a branch passes to the body wall of the dorsal 

 side, and from the connection between the tracheal trunks a 

 branch passes to the ventral body wall in the eighth and ninth 

 somites. 



3. In all forms there are six double rows of gills or buttress 

 folds within the rectum, and a similar number of rows of gill 

 bases appear on the outside. The single rows which go to make 

 up these double rows are not constant in position. They are 

 paired, either into three right and three left or three dorsal and 

 three ventral double rows, or, in some cases, apparently evenly 

 arranged and separated completely from each other, 



4. The dorsal trunks and the postdorsals always supply the 

 dorsal two-thirds of the rows of gills. This will be either three 

 full double rows and halves of two more, as in Cordulegaster, 



