336 STEPHEN G. RICH 



difference between them save that the latter has a longer anal 

 canal and hence a larger branch of the postdorsal supplying it. 

 Otherwise the description of G. descriptus will fit both species. 



The number of gill bases to a row in this form is twelve — the 

 lowest number in any form I have studied. The gills are alike 

 throughout the rectum. The gill bases are very nearly rectangu- 

 lar or oval, with the long axis pointing directly in a sagittal 

 direction; they are subopposite, as in the last genus. The 

 tracheal branches enter the gills at the same points as in Ophio- 

 gomphus. 



Internally, the rectum of this form bears a mass of villi so 

 long as to give a complete velvety covering to the walls of the 

 chamber. In no other form is this so fully the case. The gills 

 bases are mere pads of fat under an epithelial cushion, at the 

 caudal border of which the long villi, exactly like those in Ophio- 

 gomphus, are attached. There are ten to twelve villi to a gill, 

 and such of them as are not on the caudal edge are on the 

 rounding corner of the gill nearest the longitudinal muscles. 

 The caudal border of the gill is not in the least raised, nor has it 

 any cushion caudal to the villi or any trace 'of longitudinal fold 

 (fig. 22). 



Ris ('13) describes in Onychogomphus forcipatus and Gom- 

 phus pulchellus, folds similar to those in Lanthus, but covered 

 with villi. He also describes in detail a villus of Gomphus, 

 finding its tracheation a little different from what I figure. He 

 clamis to find one very large trachea and several thin ones in 

 each villus. I find no such differentiation. 



II. FAMILY LIBELLULIDAE 



In this family there are no such differences between the recta 

 of the vaf ious subfamilies and genera as are found in the Aesch- 

 nidae. The agreement in structure in the three subfamilies is 

 very great. The description of the rectum of Plathemis lydia 

 serves for the present subfamily. It is an exceedingly common 

 form and is in every way typical. 



