NO. 9 NEW CAMBRIAN TRILOBITE FAUNULE — RASETTI 9 



tus, Bolhoparia superba, Acidiscus hirdi, Litometopus longispinus, 

 and Stigmadiscus gihhosus. Presumably the spine should be con- 

 sidered homologous in all these genera, but there are no decisive 

 arguments in favor of this assumption. In Oodiscus and B at hy discus 

 the base of the spine coincides vi^ith the position of a sharp genicula- 

 tion, the portion of the posterior border distal to this point being 

 sharply bent downward and somewhat forward. In Litometopus 

 longispinus and, to a lesser extent, in Serrodiscus suhclavatus, the 

 posterior border presents an unusual feature consisting of a narrow 

 ridge separated from the main part of the border by a short, trans- 

 verse furrow. The distal end of this ridge produces a slight rearward 

 projection of the posterior margin. In one specimen of Serrodiscus 

 suhclavatus preserving two thoracic segments attached to the cepha- 

 lon, the ridge on the cephalic margin is seen to correspond to the 

 fulcrum on the anterior margin of the first thoracic segment. A simi- 

 lar feature is present at the articulation between the first and second 

 thoracic segments. In Litometopus longispinus the tooth on the pos- 

 terior cephalic border is more conspicuous. In both species the 

 "genal" spine is located more distally than these features. 



Pygidium. — The pygidia present less variety than the cephala. The 

 axis is always well defined except in one unassigned pygidium where 

 the axial furrow is barely indicated. In Acimetopus and Bolhoparia 

 there is a large, upright spine on the second axial ring. In Serrodiscus 

 suhclavatus there is a long spine on the posterior part of the unseg- 

 mented axis. In Acidiscus there are spines on several of the axial 

 rings. Border furrow and border are well differentiated in all pygidia. 

 A well-developed articulating facet is present in all genera; this is 

 the most obvious feature for distinguishing pygidia from cephala 

 in doubtful cases. Marginal spines, sometimes visible from above, 

 more often appearing as extensions of the doublure and hence ventral 

 in position, may be present. In most species the doublure is reflexed 

 as usual in trilobite pygidia. However, in Analox bipunctata and the 

 pygidium attributed to Bathy discus dolichometopus, the pygidial 

 doublure forms an almost vertical face rather than being reflexed 

 inward to parallel the dorsal exoskeleton. In neither species this 

 feature is present in the cephalon, where the doublure appears to 

 be narrow and not sharply defined, the dorsal test being gradually 

 downrolled. 



Ornamentation. — The outer surface of the test may be smooth, 

 punctate, finely granulate, coarsely granulate with granules of one 

 size (Acimetopus) or different sizes (Bolhoparia). The larger of 

 these granules seem broken at the tip and may represent the bases 



