2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
covered with long light-colored setae, most densely on the legs and 
lateral margins of the body. 
The genus Plathemis differs from its closest relative, Libellula, in 
having the head widest behind the eyes and the front margin of the 
median lobe of the labium crenulate (Needham and Betten, 1901). 
THE HEAD 
The head of Plathemis lydia (figs. 1-13) is essentially hypogna- 
thous but is slightly inclined to the horizontal so that the ventral por- 
tions are anteriormost. The head is approximately 4.5 mm. in width, 
widest behind the compound eyes, and somewhat wider than long. 
The antennae are located on the median portion of the face, above a 
protuberant transverse ridge, which is provided with numerous long 
scurfy hairs. The enormous eyes cap the prominent anterolateral 
angles of the head, while the mandibles and maxillae are lateroventral 
in position. The prehensile spoon-shaped labium, characteristic of all 
libelluloid naiads, forms, in its natural position, a mask that com- 
pletely covers the other mouthparts and the face up to the antennal 
bases. 
SUTURES OF THE CRANIUM 
The principal cranial sutures of Plathemis lydia are the epicranial, 
postoccipital, pleurostomal, epistomal, clypeolabral, clypeal, ocular, 
and antennal sutures. 
The EPICRANIAL “SUTURE” or ecdysial sulcus is completely de- 
veloped and consists of postfrontal arms and a coronal stem. This 
“suture” is more or less T-shaped, with the postfrontal arms diverging 
laterally, a condition that is typical of odonate naiads (Snodgrass, 
1947). The postfrontal sutures (pfs) or arms of the cleavage line 
are well developed but extremely short and merge with the ocular 
sutures at the posteromedial corners of the compound eyes. The 
coronal suture (cos) or stem of the cleavage line is of greater extent 
and proceeds across the top of the head almost to the cervical margin 
where it meets the postoccipital suture. 
The POSTOCCIPITAL SUTURE (pocs) is not strongly developed and 
lies very close to and somewhat parallel with the dorsal and lateral 
margins of the foramen magnum. The greatly elongated exterior 
edges of the posterior tentorial pits are partially coincident with this 
suture. There is no occipital suture. 
The PLEUROSTOMAL SUTURES (pms) are only faintly indicated 
where they are identical with the exterior edges of the anterior ten- 
torial pits formed by the fan-shaped ends of the anterior tentorial 
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