RAYMOND: NEW AND OLD SILURIAN TRILOBITES. \f 



Illaenurus Hall, 1863. 



Type, Illaenurus quadratus Hall. 



16th Rept. N. Y. state cab. nat. hist., 1863, p. 176, pi. 7. 



Although Hall believed this species to be closely allied to Illaenus, 

 as indicated by the name, it seems more probable that it belongs to 

 the Asaphidae and is allied to Symphysurus. 



Illaenopsis Salter, 1866. 

 Type, Illaenopsis thomsoi^i Salter. 

 Mem. Geol. surv. Gt. Britain, 1866, 3, p. 256. 



As has been repeatedly pointed out, the grooved pleura of the 

 thorax of this trilobite exclude it from the Illaenidae, and place it near 

 Symphysurus in the Asaphidae. 



OcTiLLAENUS Salter, 1867. 

 Type, Illaenus hisingeri Barrande. 

 Monog. Brit. Silurian trilobites, 1867, pt. 4, p. 182. 



This genus was erected by Salter to contain the type, a species in 

 which the pleura of the first thoracic segment are produced into spines. 

 There are eight segments in the thorax, the axial lobe is narrow, the 

 glabella well defined, eyes of medium size and far back, free cheeks 

 with sharp genal spines Pygidium about as long as wide, without 

 defined axial lobe. This species could probably be placed with 

 Dysplanus, but I would follow Salter in the recognition of the remark- 

 able development of spines on the first thoracic segment, it being a 

 unique example of such a characteristic among the smooth trilobites. 



Ectillaenus Salter, 1867. 

 Type, Illaenus perovalis Murchison. 

 Monog. Brit. Silurian trilobites, 1867, pt. 4, p. 182. 



Holm has pointed out that in proposing this genus Salter confused 

 the true /. perovalis of Murchison and a new species afterward de- 



