1901.] XATUKAL SCIENCES OF PHILADELPHIA. 133 



seuted by the numerator, the posterior group of teeth by the 

 denoioinator. 



The abbreviation rec. is used to indicate recurved teeth, as in 

 Pamphlebia. 



To ilhistrate tlie application of the formula, I may refer to fig. 

 13, PI. Ill, of Argia h'lpuncbi.hda. In this species thfre are six- 

 teen folds, eight larger and eight smaller. The formula would be 

 therefore: 8 (F 12-14, f 1-3), where F represents the larger 

 fields, with teeth varying in number from twelve to fourteen; f, 

 the smaller folds with teeth varying from one to three. 



The teeth here are all of approximately the same size. 



For Xaidha[/rio)i erythroneurum (fig. 15), where the teeth have 

 a definite arrangement into groups, we must use a more compli- 

 cated formula. Here there are sixteen fields, each field showing 

 two distinct groups of teeth. The anterior groups are represented 

 by the numerators of the fractions, the posterior by the denomina- 

 tors. The difference in size of the teeth is indicated by the 

 marks ' and ", the former representing the larger teeth, the latter 

 the smaller ones. 



The formula therefore is 8 (F ^_~^~, f ^'). 



The omission of one or more folds from an individual gizzard 

 is not uncommon (see fig. 12), so that often it is only possible to 

 construct a fo)"mula when several specimens of a given species are 

 examined and compared. 



The following list of the species of adults whose gizzard-armatures 

 have been studied gives the number of each sex examined, the 

 abdominal segment in which the gizzard was found (and often 

 whether in the anterior or posterior part of the segment), the 

 locality whence the material came and the armature formula: 



Sub-family CALOPTERYGIN^. 

 Legion 1. — Calopteryx Selys. 



Calopteryx maculata Beau v. 2 d^. Pennsylvania. 4 (F 6-8, 

 f 2-4, p 4-5, f 2-4). 



Calopteryx apicalis Burm. cf'. 6th segment, ant. Tom's 

 River, N. J. 4 (F 6-8, f 2-4, f 5-7, f 2-4). 



Calopteryx apicalis Burm. £ . 5th segment, ant. Tom's 

 River, N. J. 4 (F 6-8, f 2-4, f 5-7, f 2-4). 



