NO. 3 UPPER CAMBRIAN TRILOBITE FAUNAS — RASETTI 91 



Discussion. — This species is very similar to A. arsoides in all pro- 

 portions. Apart from lack of an occipital spine, it may be distinguished 

 at least statistically by the lesser divergence of the anterior section of 

 the facial suture, greater convexity of the preglabellar field, slightly 

 upsloping palpebral area, and convex instead of flat genal spine. It is 

 also very similar to A. minor, with which it had been confused before 

 large collections were made. It may be distinguished from that species 

 by the usually slanted instead of transverse course of the ocular ridge, 

 sharper anterior angles of the cranidium, slightly smaller palpebral 

 lobe, and lesser average slope of the palpebral area. The pygidia are 

 quite different, the one of A. minor being proportionately shorter and 

 wider, with narrower border and furrows extending almost to the 

 margin. The pygidium of A. inermis is more like that of A. arses. 

 The present species and A. minor occupy different stratigraphic posi- 

 tions, and no transition forms are known from intermediate beds. 



Occurrence. — The type locality is cns/2. Shields Ridge. Also present 

 in collections cnr/4, cns/4 and cnt/4. It occurs in association with A. 

 tumifrons or A. arsoides. 



Types.— Uolotype: U.S.N.M. 144669. Paratypes: U.S.N.M. 

 144670-1. 



APHELASPIS TUMIFRONS Resser 



Plate 19, figures 1-7 

 Aphelaspis tumifrons Resser, 1938a, p. 60, pi. 13, fig. 15. 



Available material. — The hypodigm includes the holotype and sev- 

 eral other cranidia in fair state of preservation. Abundant and well- 

 preserved new specimens from various localities allow a more complete 

 description. 



Description. — Glabella poorly defined on upper surface by a very- 

 shallow axial furrow. Occipital furrow shallow ; occipital ring bearing 

 a node. Frontal area averaging 0.45 times the glabellar length. Pre- 

 glabellar field with definite median boss. Border defined by somewhat 

 gradual change in slope, of average width medially, almost vanishing 

 at the sides. Anterior cranidial margin slightly pointed medially. Pal- 

 pebral area approximately horizontal, about 0.3 times the glabellar 

 width ; palpebral lobe more or less upsloping, not defined by palpebral 

 furrow on outer surface. Average exsagittal length of palpebral lobe 

 0.4 times the glabellar length and 0.8 times the distance to posterior 

 margin. Ocular ridges wide, almost undefined on outer surface, some- 

 what slanted backward. Transverse width of posterior area 0.8 times 

 width of occipital ring. Free cheek with somewhat convex ocular plat- 

 form, broad and vaguely defined border furrow, and fairly long, 

 cylindrical genal spine. 



