NO. 3 UPPER CAMBRIAN TRILOBITE FAUNAS — RASETTI 107 



dance of the material and of the almost identical association of cranidia 

 and pygidia observed by the author in other areas. Then, if Palmer's 

 pygidium is correctly attributed, we must conclude that there are two 

 genera of trilobites with indistinguishable cranidia and very different 

 pygidia, not an unusual situation. In that case it would be impossible 

 at present to decide which one is Syspacheilus, since there is no reliable 

 assignment of a pygidium to the type species. 



Whatever the solution of this problem may be, the author sees no 

 reason to maintain the use of Syspacheilus for the species with a short 

 pygidium, typified by Syspacheilus praecedens, which are almost iden- 

 tical with Metisia metisensis (Rasetti, 1963). If these forms are 

 sufficiently distinct from Modocia oweni to warrant generic recogni- 

 tion, then they should be referred to Metisia accepted as a valid genus. 

 Possibly the same arguments apply to Talbotina (at least as repre- 

 sented by T. juweli Lochman, in Lochman and Duncan, 1944) and 

 Ithyektyphus Shaw (1956), (Lochman and Hu, 1960). 



In conclusion, it seems best at present to restrict the use of Sys- 

 pacheilus to the type species until the characters of the genus are 

 clarified. 



MODOCIA DUBIA (Resser) 

 Plate 1, figures 22-26 



Asaphiscus? agatho Walcott (part), 1916b, p. 391, pi. 62i, fig. 9a (only). 



Ehmania dubia Resser, 1938a, p. 75, pi. 9, figs. 18, 19. 



Uncaspis tennesseensis Resser, 1938a (part), p. 105, pi. 9, fig. 21 (only). 



Three species of Modocia are common in the red beds at the base 

 of the Nolichucky, and two of them were also collected at various 

 localities in the underlying beds of the Maryville formation. Through 

 association in several instances it was possible to obtain an unambigu- 

 ous assignment of the pygidia to the respective cranidia. 



Description. — Entire cranidium of moderate convexity. Glabella 

 moderately convex, proportionately wide and short, strongly tapered, 

 rounded in front, bearing a trace of furrows on outer surface. Pregla- 

 bellar field flat ; border not greatly elevated or convex, about as wide 

 (sag.) medially as the preglabellar field. Palpebral area slightly convex 

 and downsloping; ocular ridges faint; palpebral lobes one-third the 

 glabellar length, placed about parallel to the axial furrow, defined by 

 a sharp palpebral furrow, situated well in advance of glabellar mid- 

 point. Posterior area long (exsag.), deeply furrowed. The anterior 

 sections of the facial sutures are barely divergent. The surface of the 

 test is densely and finely granulate. Length of larger cranidia 12 mm. 



Pygidium twice as wide as long. Axis stout and prominent, showing 

 two distinct rings, and in some specimens a less distinct third ring. 



