No. 1. — New and old Silurian Trilobites from Southeastern Wisconsin, 

 with Notes on the Genera of the Illaenidae. 



By Percy E. Raymond. 



In the F. H. Day collection, received in January 1881, as a gift of 

 Mr. Alexander Agassiz, the Museum of Comparative Zoology secured 

 one of the three great collections of the Silurian fossils of southeastern 

 Wisconsin. It is particularly rich in Illaenidae, but contains also 

 such rare forms as Harpes telleri, Trochurv^ nasutus, and Dicrano- 

 peltis telleri, to be found elsewhere only in Mr. Teller's magnificent 

 collection. 



The trilobite fauna of the quarries around Milwaukee and Racine 

 differs considerably from that found in the vicinity of Chicago, so 

 that some of the species described by Weller from the latter area are 

 either absent from the M. C. Z. collection or represented by speci- 

 mens from other sources than the Day collection. The Phacopidae 

 have been omitted from the present study, and only such species are 

 mentioned as are represented by specimens which add something to 

 what has already been published. 



ILLAENIDAE Hawle and Corda. 



The Illaenidae form a remarkably homogeneous group, and in spite 

 of the great number of species which have been described only three 

 generic or subgeneric names (Illaenus, Bumastus, and Thaleops) are 

 in common use. Holm recognized only Illaenus and Bumastus and 

 other writers have been even more conservative, referring all the 

 species to Illaenus. The only serious attempt to subdivide the genus 

 is that made by Salter, who recognized eight subgenera (including 

 Illaenus), but none of Salter's names has ever come into general use, 

 although some of them could be adopted advantageously. In all, 

 seventeen subgenera or genera have been proposed for inclusion in 

 this family, but only seven of these seem to be valid. The names, in 

 chronological order, are: — Cryptonymus Eichwald 1825, Illaenus 

 Dalman 1826, Deucalion Shtsheglov 1827, Bumastus Murchison 

 1839, Archegonus and Dysplanus Burmeister 1843, Thaleops Conrad 

 1843, Alceste Hawle and Corda 1847, Rhodope Angelin 1854, Actino- 



