NO. 3 UPPER CAMBRIAN TRILOBITE FAUNAS — RASETTI 79 



Pygidium 2.3 times as wide as long. Anterior margin definitely 

 curving backward. Axis elevated, tapered, showing 3 rings plus a 

 terminal section. Pleural lobes with distinct furrows and shallower 

 interpleural grooves even on outer surface. Border flat, narrower 

 medially ; margin with slight median inbend. 



Length of largest cranidium about 15 mm. 



Discussion. — This species may be compared with A. laxa and A. 

 walcotti. From laxa it differs consistently in the somewhat more con- 

 vex glabella, less bulging preglabellar field, more posterior position 

 of the eyes, and shorter (tr.) posterior area. The pygidium is propor- 

 tionately narrower and more strongly furrowed. Compared with A. 

 walcotti, A. quadrata has a cranidium with greater relief, lesser rela- 

 tive sagittal length of border relative to preglabellar field, and the 

 pygidium has a more curved anterior margin. 



Occurrence. — The type locality is U.S.N.M. 120, Shields Ridge. 

 Although material under this label includes collections from different 

 horizons, the association of the species with A. camiro and A. laxa is 

 proved by the occurrence of the three species on the same piece of 

 rock. Also collected by the author at localities cns/15, cns/16, cnr/20 

 and cnrV20, in association with the equally common A. camiro and 

 rare specimens of A. laxa. 



Types. — Holotype and paratypes: U.S.N.M. 94924. Plesiotypes: 

 U.S.N.M. 144637-8. 



APHELASPIS TARDA Rasetti, new species 



Plate 20, figures 1-18 



Available material. — Large numbers of cranidia, free cheeks, and 

 pygidia from several localities, both preserving the test and exfoliated. 



Description. — Glabella of average shape and convexity; occipital 

 ring bearing a median node. Frontal area averaging 0.65 times the 

 glabellar length, but showing wide variation in this ratio, uncorrelated 

 with size, locality, or horizon. Preglabellar field more or less convex 

 longitudinally ; border flat, forming sharp angle with preglabellar field. 

 Sagittal length of border on average half of that of preglabellar field ; 

 this ratio is also highly variable. Palpebral area and lobe always more 

 or less upsloping; palpebral area from 0.3 to 0.4 times as wide as 

 glabella. Ocular ridges transverse to slightly slanted backward. Length 

 of palpebral lobe about 0.4 times glabellar length and equal to distance 

 to posterior margin. Width (tr.) of posterior area 0.8 times the width 

 of occipital ring. 



Free cheeks with border furrow distinct and border of variable 

 width, flat to somewhat convex. Genal spine subcylindrical, rather flat 

 on upper side, wide at the base, of moderate length. 



