76 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 148 



APHELASPIS WALCOTTI Resser 



Plate 18, figures 10-20 



Aphelaspis zvalcotti Resser, 1938a, p. 59, pi. 13, fig. 14, 



Aphelaspis walcotti Resser, Palmer, 1962b, p. 33, pi. 4, figs. 24, 28, 33. 



Aphelaspis simulans Resser, 1938a, p. 59, pi. 13, figs. 19-21. 



Available material. — The hypodigm of A. walcotti consists of sev- 

 eral cranidia, free cheeks, and pygidia, moderately well preserved and 

 exfoliated. The types of A. simulans also include cranidia, free cheeks, 

 and pygidia. Abundant material from the author's collections at various 

 localities is also available. 



Description. — Cranidium with less relief than average in the genus. 

 Glabella defined by a shallow axial furrow on outer surface, weakly 

 convex. Preglabellar field moderately downsloping and somewhat 

 convex. Border fiat, forming sharp angle with preglabellar field. 

 Frontal area on average 0.6 times the length of glabella plus occipital 

 ring. Sagittal length of border averaging slightly less than the length 

 of preglabellar field. Palpebral area of average width, from slightly 

 upsloping to horizontal. Ocular ridges very faint, almost transverse 

 to slightly slanted backward. Palpebral lobe averaging somewhat more 

 than one-third the length of glabella plus occipital ring ; distance from 

 posterior end of palpebral lobe to posterior margin greater than length 

 of palpebral lobe. Anterior section of facial suture with average diver- 

 gence ; anterior angles of cranidium more widely rounded than in most 

 species. Width (tr.) of posterior area less than width of occipital ring. 



Free cheek with ocular platform of low convexity. Border slightly 

 concave, upturned, not defined by furrow or sharp change in slope. 

 Genal spine flat on upper side, of less than average length, rather 

 rapidly tapered. 



Pygidium 3 times as wide as long. Anterior margin almost straight ; 

 anterior angles sharp. Axis short, showing 2 or 3 rings plus a terminal 

 section. Pleural lobes flat, with furrows and interpleural grooves, 

 almost indistinct on outer surface, well marked on internal impression. 

 Border flat, very narrow medially. 



Length of largest cranidium collected 15 mm. 



Discussion. — Careful examination of the types of Aphelaspis simu- 

 lans and additional topotype material preserved in the U.S. National 

 Museum indicates that the species may be definitely synonymized 

 with A. walcotti. Aphelaspis simulans is based on a population show- 

 ing a slightly greater relief of the cranidial parts than the average in 

 A. walcotti, in particular, definitely upsloping palpebral area; however, 

 the differences are not consistent among individual cranidia and fall 

 within the range of variability observed within a single population. 

 The pygidium also fully agrees with that of typical A. walcotti. 



