80 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 148 



Pygidium 2.5 times as wide as long. Axis showing 2 or 3 distinct 

 rings on outer surface. Anterior outline of pleural lobes gradually 

 curving backward without sharp geniculation, anterior angles more 

 or less rounded. Pleural platforms with very faint furrows on outer 

 surface ; border furrow and border indefinite. 



Length of largest cranidium 18 mm. Length of largest pygidium 

 4 mm., width 10 mm. 



Discussion. — The numerous populations of this species observed 

 show individual variability, especially in the relative proportions of 

 border, preglabellar field, and glabellar length. Single cranidia with 

 short preglabellar field and long border may be confused with A. wal- 

 cotti and possibly other species. However, statistically the species is 

 fairly well defined. The considerable length of the preglabellar field 

 in most individuals, the upsloping palpebral area, definite flat border 

 and border furrow on the free cheeks, and the characteristic pygidium 

 with well-rounded anterior angles clearly distinguish a population of 

 A. tarda from one of A. walcotti. The two species are rather widely 

 separated stratigraphically, and no transition forms were observed in 

 intermediate beds. 



Occurrence. — Common at every locality where fossiliferous beds 

 of the upper Aphelaspis zone are developed. The type locality is 

 cnw/20, Washburn. Also present in collections cnu/1, cnv/1, cnw/1, 

 cnw/14, cnu/15, cnv/15, cnx/20, cnu/21, cnv/21 and Oder's collec- 

 tion No. 14A. The species is also common in beds cnu/7, cnv/7 and 

 cnvV7 in the Purchase Ridge section, Scott County, Va. 



Ty/'^^.— Holotype : U.S.N.M. 144639. Paratypes: U.S.N.M. 

 144640-2. 



APHELASPIS LAXA Resser 



Plate 12, figures 18-21 ; plate 13, figures 8-15 



Aphelaspis laxa Resser, 1938a, p. 60, pi. 13, fig. 18. 



Available material. — The holotype is an exfoliated cranidium lack- 

 ing the palpebral lobes and part of the anterior border. Several para- 

 type cranidia are much better preserved, and further topotype material 

 exists in the U.S. National Museum collections. The species also 

 occurs in several of the author's collections, represented by cranidia, 

 free cheeks, and pygidia. 



Description. — Glabella of low convexity, rising little above the level 

 of the cheeks, but defined at least laterally by a rather deep axial 

 furrow. Frontal area about 0.6 times the glabellar length. Preglabellar 

 field strongly convex longitudinally, hence sloping down steeply to 

 border furrow. Border almost flat, horizontal ; sagittal length of border 



