NO. 9 NEW CAMBRIAN TRILOBITE FAUNULE — RASETTI II 



be difficult to carry out because genera that are alike in one feature 

 are apt to be strikingly different in one or several others. For this 

 reason the genera are described herein in alphabetical order. 



ACIDISCUS Rasetti, new genus 



Description. — Cephalon of moderate convexity, semielliptical. 

 Glabella not reaching the anterior border, subcylindrical, with two 

 short, somewhat pit-like pairs of lateral furrows, in the two known 

 species possessing a small upright spine on its posterior portion. 

 Occipital furrow wide; occipital ring well defined, bearing a spine. 

 Posterior border straight from the axial furrow to the base of the 

 genal spine. Lateral cephalic border with one or two pairs of mar- 

 ginal spines. 



Pygidium with long, multisegmented axis. Geniculation situated 

 rather distally; facet well developed. Pleural regions unfurrowed; 

 border and border furrow well developed, of average width. Surface 

 smooth or with weak ornamentation. The narrow doublure may 

 be extended into short spines. 



Type species. — Acidiscus birdi Rasetti, new species. 



Occurrence. — Late Lower Cambrian of New York. Also in the 

 Purley Shales of Warwickshire, England (Rushton, private com- 

 munication). 



Discussion. — This genus is obviously a close relative of Serrodis- 

 cus. The pygidium is much the same as in that genus. The cephalon, 

 however, has important distinctive features, the most significant being 

 the development of glabellar and occipital furrows. Another distinc- 

 tive character is the tendency to develop spines on the glabella, 

 occipital ring, at the genal angle, and on the cephalic border. The 

 glabellar furrows also indicate relationship to Stigmadisc%s; in sev- 

 eral features Acidiscus may be considered transitional between the 

 latter genus and Serrodiscus. 



ACIDISCUS BIRDI Rasetti, new species 

 Plate 1, figure 2 ; plate 6, figures 11-19 



Available material. — Numerous cephala and pygidia, of which a few 

 are fairly complete. 



Description.— Co^h^Xon semielliptical. Glabella rising well above 

 the cheeks, very convex transversely, tapered in anteriormost por- 

 tion, narrowly rounded in front, occupying with occipital ring more 

 than two-thirds of cephalic length. Two pairs of distinct, short, 



