124 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



Observations. — There are many species of the genus Wilbernia 

 and all parts of the tests are known, but only the cranidium of the 

 type species has been definitely identified in the preliminary study 

 of the Texas collections. The best known species of the genus 

 is Hall's Ptychoparia diademata from the Franconia formation of 

 Wisconsin. 



WILBERNIA PERO (Walcott) 



Plate 15, figs. 22, 23 



Ptychoparia pcro Walcott, 1890, Proc. U. S. Nat. Mus., Vol. 13, p. 274, pi. 21, 



fig. 6. (Described and illustrated.) 

 Anomocare pero Walcott, 1912, Monogr. U. S. Geol. Surv., Vol. 51, p. 212. 



(Generic reference.) 

 Wilbernia pero Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, No. 2, p. 60, 



pi. 13, fig. 4. 



More material of this species will be available, after the collections 

 from the type, locality are more fully studied. 



Formation and locality. — Upper Cambrian: (70) Wilberns for- 

 mation. Morgans Creek, Burnet County, Texas. 



Genus XENOSTEGIUM Walcott 



Xenostegium Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, No. 2, p. 60. 



The genus Xenostegium includes a number of trilobites which have 

 long been known, usually being assigned to Megalaspis. 



The cranidium expands in front and often at the same time in- 

 creases in convexity. It is distinguished by this swelling rather than 

 by any definite dorsal furrow. Border always present, usually a 

 simple concave rim. Glabellar furrows absent. Occipital furrow 

 absent unless represented by two pits. Fixed cheeks narrow. Pal- 

 pebral lobes usually more or less erect. Free cheeks undetermined. 



This type of pygidium, generally placed in the genus Megalaspis, 

 is characterized by its triangular outline and median posterior spine ; 

 the axis is often not delimited at the posterior end by the dorsal 

 furrow as it, after outlining the axis along the sides, turns slightly 

 outward and usually disappears, a slight swelling carrying the axis 

 back to the base of the terminal spine. A flattened border is defined 

 by a shallow depression between it and the slope of the pleural lobe. 

 Several transverse furrows are usually developed sufficiently to permit 

 the recognition of several segments. Very faint pleural furrows are 

 occasionally outlined at the anterior end. 



