NO. 7 PTYCHOPARIID TRILOBITES — RASETTI 29 



front and at the sides. Ocular ridges first directed forward and 

 curving backward. Palpebral lobes poorly differentiated within the 

 lateral vertical slope, situated near the level of the glabellar midpoint 

 (P. yorkensis) or farther back (P. roddyi). Posterior limb not 

 extending laterally beyond the palpebral area, with a deep, wide fur- 

 row not reaching the end of the limb. Anterior facial sutures strongly 

 convergent in front of the eyes ; frontal portion somewhat ventral- 

 intramarginal. Posterior branch first directed inward, then slightly 

 outward, and inward again before reaching posterior margin. Free 

 cheeks very narrow, dorsally consisting of wide, convex border and 

 small ocular platforms. Test thick; upper surface granulated. 



Type species. — Periommella yorkensis Resser. 



Remarks. — Lochman (1947) gave a diagnosis based on material 

 described herein. Notwithstanding the aberrant features, the simi- 

 larity in the structure of the glabella, frontal area, and posterior limbs 

 indicates relationship to Periomma. 



PERIOMMELLA YORKENSIS Resser 



Plate I, figures 8, 9; plate 5, figures 1-8 



Periommella yorkensis Resser, Geo!. Soc. Amer. Spec. Pap. 15, p. 93, pi. 3, 

 figs. 35, 36, 1938. 



Available material. — The type lot consists of the holotype and two 

 paratypes, all poorly preserved as internal casts in weathered sand- 

 stone. Numerous excellent cranidia and a few free cheeks in limestone 

 are extant. 



Description. — Features included in the generic diagnosis are omitted. 

 Glabella with a trace of four pairs of furrows visible on whitened 

 specimens. Relative lengths (sag.) of preglabellar field and border 

 about equal in average cranidia 6 mm. long. Marginal furrow straight, 

 deeper at the sides, shallow medially on account of a median preglabel- 

 lar boss that becomes more pronounced in larger individuals. Border 

 very convex, slightly arched transversely, underfolded in front so that 

 the frontal portion of the facial suture has a ventral course. Palpebral 

 area flat and slightly upsloping, but not reaching the level of the 

 glabella; ocular ridges directed outward and forward from the gla- 

 bella, wide and poorly defined on the upper surface, turning backward 

 in a regular curve and fading out along the margin of the sharp 

 vertical drop of the palpebral area along its anterior edge. Palpebral 

 area also turned down vertically at the sides, so that its lateral margin 

 is concealed in dorsal view. Palpebral lobes hardly differentiated as 

 the marginal portion of the downfolded palpebral area, invisible from 



