334 STEPHEN G. RICH 



nine gill bases 45° from the transverse line. . Most typically 

 gomphine is the way in which the tracheae pass into the gill bases 

 of this form: they do not enter in the center, but pass in along 

 the two edges. 



Within the rectum the distribution of vilU corresponds to this 

 tracheation. The viUi of each gill are clustered in a row along 

 one curving side of the gill base ; this is the side pointing caudad 

 and towards the muscle band. The cushion on the cephahc side 

 of the gill, compared to that of Hagenius, has become enlarged 

 and flattened to an oval patch, while that on the caudal side 

 is much reduced. The fat in the gill is all under the cephahc 

 cushion. The villi do not taper and are the longest as yet seen; 

 they bear a single seta on the conical tip and one or two on the 

 sides near the tip. 



Ophigomphus has a remnant of the longitudinal fold of the 

 Aeschninae in the form of a ridge connecting the raised caudal 

 edges of two rows of gills. Figure 19 shows this; figure 20 shows 

 the caudal aspect of a gill, and figure 21 a villus. 



Gomphus has a rectum essentially the same as in Ophiogom- 

 phus. The regularity of distribution of tracheal branches is here 

 perhaps a Uttle more perfect, but the two recta are not distin- 

 guishable externally. The appearance of the gill bases is shown 

 in figure 16. In this genus I have dissected the two species, 

 Gomphus descriptus (figs. 16 and 21) and G. villosipes. I find no 



Fig. 16 Ventral view of rectum of Gomphus descriptus. 



Fig. 17 Gill of Hagenius brevistylis. 



Fig. 18 Villus and tracheation from gill of same. 



Fig. 19 Cephalic aspect of gills of Ophiogomphus carolus, showing also 

 arrangement of parts on inner surface of rectum. 



Fig. 20 Caudal aspect of gill of same, villi lifted up. 



Fig. 21 Villus of same. 



Fig. 22 Gills of Gomphus descriptus. 



Fig. 23 Distribution of tracheae to gills in Leucorhinia intacta. 



Fig. 24 Dorsal view of rectum of Plathemis lydia. 



Fig. 25 Ventral view of same. 



Fig. 26 Gill of same. 



Fig. 27 Arrangement of gills of same. 



Fig. 28 Schematic cross-section of rectum of Libellula sp. (From a MS. 

 drawing by Miss E. Andrews.) 



Fig. 29 Gill of Didymops transversa. 



