22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
or anal pyramid (Tillyard, 1917). The cerci are considered to be the 
appendages of the eleventh segment. The second group is believed by 
Heymons (1904) to be the remnant of the twelfth abdominal segment. 
It is composed of the laminae supra-anales and the laminae sub-anales. 
The Eprproct (ep) is the tergum of the eleventh segment (Hey- 
mons, 1904; Crampton, 1918). In Plathemis lydia the epiproct is a 
wide, triangular, horny process that emerges from beneath the postero- 
dorsal margin of the tenth tergum to surround the anus from above. 
The PARAPROCTS (pp) complement the epiproct by surrounding the 
anus laterally and ventrally. Each of these two triangular horny 
processes resembles the epiproct in appearance but is slightly smaller 
in size. The paraprocts, according to Heymons (1904), are actually 
the true cerci. Crampton (1918) contended that the paraprocts and 
cerci were distinct structures but that they belonged to the tenth seg- 
ment. The most current theory is that of Snodgrass (1931), who 
maintains that the paraprocts pertain to the eleventh segment and 
probably represent the sternal sclerites of that segment. 
The cErcI (ce) are paired appendages that arise from the dorso- 
lateral posterior margin of the tenth tergum, immediately laterad of 
the epiproct. These appendages were termed “cercoids” by Heymons 
(1904) and Tillyard (1917), both of whom maintained that the true 
cerci were the lateral anal appendages or paraprocts. In recent papers, 
both Snodgrass (1954) and Asahina (1954) agree that the “cercoids” 
of Heymons are the true cerci, as previously maintained by Crampton — 
(1918). Crampton, however, theorized that the cerci arose from the | 
tenth abdominal segment, but more recent authors (Snodgrass; 
Asahina) place them with the epiproct and paraprocts on the eleventh 
segment. 
If the epiproct and paraprocts are spread apart, as in figure 23, four | 
small sclerites can be seen surrounding the anus (an). According to © 
Heymons (1904), the middorsal pair or laminae supra-anales (sa) is — 
the reduced twelfth tergite, while the two lateral ones or laminae sub- | 
anales (la) represent the bipartite sternite. The membranous cir- 
cumanal fold containing these sclerites has been termed the periproct © 
by Snodgrass (1931). | 
COMPARATIVE ANATOMY OF THE GENUS PLATHEMIS | 
The present writer has found that the mature naiads of the two — 
known species of the genus Plathemis are practically identical as re- | 
gards external morphology. However, they have been shown by | 
Needham and Westfall (1955) to differ in the relative size and — 
