NO. 9 NEW CAMBRIAN TRILOBITE FAUNULE — RASETTI I3 



ACIDISCUS HEXACANTHUS Rasetti, new species 

 Plate 7, figures 1-6 



Available material. — Two cephala and one pygidium. 



Description. — It is sufficient to point out the differences from the 

 type species. Shape and convexity of cephalon and pygidium the 

 same as in ^. birdi. Glabella and occipital ring, including the spines, 

 almost identical. Cephalic border bearing, in addition to an anterior 

 pair of spines as in A. birdi, a second pair halfway between the first 

 pair and the genal angle. Posterior cephalic border and genal spine 

 as in preceding species. Lateral border lacking tubercles. 



Pygidium (referred to the species on account of the ornamentation) 

 with entirely segmented axis, also showing 12 rings including the 

 terminal section. Axial spines developed on the first six instead of 

 seven rings. The character of the pygidial doublure has not been 

 ascertained. 



Surface of cheeks and pleural regions of pygidium covered with 

 small, sparse, but sharply elevated granules. The ornamentation is 

 less marked in the holotype cephalon than in the paratype cephalon 

 and pygidium. Similar individual variations in the identical type of 

 ornamentation were observed in Oodiscus subgranulatus. 



Occurrence. — Collection cs-4, North Chatham. 



7y^^^._Holotype : U.S.N.M. 145989. Paratypes: U.S.N.M. 

 145990. 



ACIMETOPUS Rasetti, new genus 



Description. — Cephalon and pygidium strongly convex. Glabella 

 divided by a deep, wide transglabellar depression into anterior and 

 posterior lobes. Anterior lobe bulbous, not reaching the anterior 

 border. Transglabellar depression subdivided laterally into two pairs 

 of furrows, isolating a pair of small lobes ; these furrows are confluent 

 medially into one broad furrow. Posterior glabellar lobe strongly 

 elevated and extended into a long, uptilted spine. Occipital ring short 

 (sag.), well defined by occipital furrow, not spinose, partly concealed 

 in dorsal view by the rearward extension of the glabella. Cheeks 

 strongly convex, confluent anteriorly without any trace of a pre- 

 glabellar depression, in their posterior portion overhanging the bor- 

 der furrow. Border well developed, defined by a deep border furrow, 

 approximately lying in a plane, wide anteriorly, tapering toward the 

 genal angle, extended into a pair of small lateral spines. Near the 

 genal angle the lateral border furrow is confluent with the deep 

 posterior border furrow ; the border at this point is greatly reduced. 



