Io SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
teriorly produced, forming a small triangular lobe. The remainder of 
the distal margin is evenly crenulate and possesses 10 or 11 short, 
spiniform setae (lgs) on each side of the central lobe. 
The prementum (prm) bears, on each side, eight long, slender setae 
(prs), of which the five outermost ones are distinctly the longer. In 
addition, a pair of small secondary setae is present on each side just 
mesad of the premental setae with which they are often included by 
many authors. 
Of the numerous setae mentioned above, the lateral palpal setae 
(Ips) and the premental setae (prs) are of the greatest importance in 
the current taxonomy of immature Odonata. 
The postmentum (psm) is a hollow stalk that supports the pre- 
mentum, and the entire labium swings on its base. The lateral margins 
of the base of the postmentum are thickened and extend laterally in 
the head membrane as a pair of folding articular rods (al) the ends 
of which lie adjacent to the maxillary cardines. “The true hinge 
points of the labium on the head are thus at the mesal ends of these 
rods where the rods join the basal lobes of the postmentum” (Snod- 
grass, 1954). 
CERVIX OR NECK 
Since the origin of the cervix (figs. 2, 16) has not been definitely 
established, it is discussed briefly and separately from both head and 
thorax. In Plathemis lydia this membranous region bears two pairs 
of sclerites, the lateral cervical sclerites and the dorsal cervical 
sclerites. 
The LATERAL CERVICAL SCLERITES (Isc) are large, laterally pro- 
tuberant pyramidal sclerites on the anterolateral margin of the neck 
membrane. 
The DORSAL CERVICAL SCLERITES (dsc) lie free in the neck mem- 
brane. They are transversely elongate and much smaller than the 
above. 
THORAX 
The thorax of Plathemis lydia (figs. 14-19) is “characterized by 
three special features. First is the obliquity of the mesothoracic and 
metathoracic pleura as indicated by the posterior slant of the pleural 
sulci from the leg bases to the wings. Second is the almost complete 
union of the adjoining pleural plates of the wing-bearing segments, 
resulting in the suppression of an intersegmental groove between the 
epimeron of the mesothorax and the episternum of the metathorax. 
Third is the dorsal extension of the upper plates of the mesothoracic 
