NO. 5 CAMBRIAN TRILOBITES 405 



guishing it from Illcenus. He did not compare it with Symphysurus 

 Goldfuss, which has a somewhat quadrate cranidium as represented 

 in its genotype Asaphus palpebrosus Dalman/ 



Dr. W. C. Brogger described a new species of trilobite from a 

 cephalon and pygidium, which he named Symphysurus iiicipieiis,' 

 that is evidently congeneric with Symphysurus palpebrosus. He also 

 describes (p. 60) and illustrates (pi. iii, figs. 9-1 1) another species, 

 S. aitgtistattts Sars and Boeck, that appears to be congeneric with 

 Hall's genus Illccmirus. This brings up the question of the distinction 

 between Symphysurus and Illccnurus. 



The genotype of Symphysurus, S. palpebrosus, appears to be a 

 form nearer to Asaphus than to Illcenus. It has a distinctly marked 

 cranidium with large eyes and a glabella expanded anteriorly. The 

 thorax has eight segments and the pygidium has a distinct median 

 lobe. 



In contrast with Symphysurus, Illcvnurus more nearl\' resembles 

 Illcenus. The facial sutures in front of the eyes are subparallel to the 

 longitudinal axis of the body; there are 10 segments of the Illcenus 

 type in the thorax and a transverse pygidium with only a slight indi- 

 cation of a central lobe. The eye of Illcvnurus is also smaller and less 

 prominent, and the free cheek is proportionally larger. 



Illcenurus should be compared with Psilocephalus Salter,^ although 

 the latter differs in cranidium, thorax and pygidium from Illcenurus. 



Illcenurus appears to be the progenitor in late Upper Cambrian 

 time, and early post-Cambrian, of Illcenus, a genus that obtained a 

 great development in Ordovician and Silurian times. 



In xA.merica there are no other species that appear to be congeneric 

 with Illcenurus quadratus. 



Among species previously referred to Ilhrnurus the following are 

 now placed under other genera : 



Illcenurus ca;;r;;.y Walcott. igos = Tsiiiania Walcott. 



Illcenurus ceres Walcott, 1905 = Tsinania Walcott. 



Illcenurus columbiana Weller, igo^=:Platycolpus Rajmond, 1913. 



Illcenurus convexa Whitfield, 1878 = Platycolpus Raymond. 1913. 



Illcenurus ? dia Walcott, ? = ? 



Illcenurus dictys Walcott, 1905 = Tj-nmH/a' Walcott. 



Illcenurus eurekensis Walcott, ? =^ Platycolpus Raymond, 1913. 



Illcenurus spp. a and b Walcott,* igi^^ Asaphus/ 



'^ Ueber die Palaeaden, Trilobiten, Niirnberg, 1828, p. 48, pi. 4, figs. 2a- 

 ^ Sil. Etagen 2 und 3, Kristiania, 1882, p. 58, pi. i, figs, i, 2. 

 ' Mem. Geol. Surv. Great Britain, Vol. 3, 1866, p. 315, pi. 6, figs. 9-12. 

 * Smithsonian Misc. Coll., Vol. 57, 1913, p. 336. 



