lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I28 



Antagmus is difficult to separate from Onchocephalus and Piasella, 

 as intergrading forms between all these three genera are known and 

 the boundaries must be set arbitrarily. Antagmus is chiefly charac- 

 terized by the strong convexity of the separate cranidial parts, median 

 expansion of the border, and divergence of the anterior facial sutures. 

 Lochman (1947) attributed much importance to the relative trans- 

 verse width of the palpebral area, a character which is affected by 

 individual variability, stage of growth, and manner of preservation. 

 The writer is more inclined to consider the divergence of the anterior 

 facial sutures as the main diagnostic feature that separates Antagmus 

 from Onchocephalus. Pia:;clla differs from Antagmus mainly in the 

 lack of a considerable medial expansion of the border, definitely down- 

 sloping palpebral area, and relatively wider (tr.) posterior limbs. 



ANTAGMUS GIGAS Rasetti, new species 



Plate I, figure i ; plate 2, figures 1-8 



Available material. — About a dozen cranidia, most of which are 

 fragmentary. 



Description. — Entire cranidium of considerable convexity. Glabella 

 defined by a deep dorsal furrow at the sides, a shallower furrow in 

 front, convex in both directions, moderately tapered, with slightly con- 

 cave sides, rounded in front. Four pairs of glabellar furrows visible, 

 the first pair shallow and short, the other three well impressed. Oc- 

 cipital furrow deep; occipital ring short (sag.), bearing a small node. 

 Frontal area sloping down steeply at the sides ; marginal furrow deep 

 at the sides, shallower medially where it bends backward to form an 

 obtuse angle and greatly reduce the preglabellar field. Border convex 

 laterally, flatter and expanded backward medially. Palpebral area 

 rising sharply above the dorsal furrow, especially in exfoliated speci- 

 mens, convex transversely, on the average horizontal, 0.6 to 0.7 times 

 the width of the glabella at its midpoint; possibly of greater rela- 

 tive width in smaller cranidia. Ocular ridges wide and low, almost 

 straight. Palpebral lobes slightly less than one-third the glabellar 

 length, slightly elevated, situated slightly back of the level of the 

 glabellar midpoint ; palpebral furrows shallow. Posterior limbs 

 slightly wider (tr.) than the occipital ring; marginal furrow wide and 

 deep, reaching the distal end of the limb. Anterior facial sutures 

 divergent in front of the eyes in dorsal view, convex outward, curv- 

 ing gradually inward and crossing the margin well at the sides ; frontal 

 portion slightly ventral. Posterior branch directed almost straight 

 outward and backward, curving definitely backward only near the 

 margin. 



