1913] The Ottawa Naturalist. 101 



ON THE GENERA OF THE EODISCIDAE. 



By Percy E. Raymond. 



It has long been known that the name Microdiscus, in com- 

 mon use for small trilobites of Lower and Middle Cambrian age, 

 is untenable, the genus Microdiscus having been founded by 

 Emmons on a young specimen of Cryptolithus (Trinucleus) . The 

 name Eodiscus was suggested in manuscript by Professor Hartt 

 when he was describing the Middle Cambrian fossils found near 

 St. John, New Brunswick, for Dawson's Acadian Geology, but 

 on the advice of Billings, he replaced it by Microdiscus at the 

 time of publication. This name was, however, mentioned in 

 Walcott's paper on the fauna of the Middle Cambrian (1884), 

 and in 1896 Matthew used the name for a section of the genus 

 Microdiscus. 1 Recently the writer proposed to raise the section 

 name Eodiscus to generic rank, replacing Microdiscus. The type 

 suggested w r as Microdiscus schucherii Matthew, which was the 

 first species listed by Matthew under his section Eodiscus.- It 

 seems wise, however, not to apply the term Eodiscus in too broad 

 a sense, but to use it in much the same way in which Matthew 

 did. 



In reviewing the various species which have been referred 

 to Microdiscus, omitting the M. quadrico status of Emmons, we 

 find three distinct types of shields. The first and most common 

 is that typified by Eodiscus schucherti or E. punctatus, in which 

 the glabella and axial lobe of the pvgidium are both definitely 

 defined by the dorsal furrows, and the cheeks of the cephalon 

 and the pleural lobes of the pvgidium are smooth. A second 

 type of shield is seen in forms like Microdiscus lobaius or M. 

 dawsoni, where the dorsal furrows are strong on both shields, 

 and the pvgidium has well defined ribs on the pleural lobes. 

 To a part of this second section Matthew gave the name 

 Dawsonia, a name which had been used in manuscript by Hartt 

 for his Microdiscus dawsoni, and which had appeared in print 

 in the Second Edition of the Acadian Geology, though not used 

 as a generic name. This section name of Matthew's would also 

 be raised to generic rank were it not for the fact that the name 

 is no longer available, having been applied by Nicholson in 1872 

 to certain bodies found associated with graptolites. I therefore 

 propose Goniodiscus for this section, with Agnostus lobaius Hall 

 as the type. 



1. Araer. Geologist, 1896 , vol. 18, p. 29, and Trans. N.Y. Acad. Sci. vol. 



15, p. 237, 1896. 



2. Ottawa Naturalist. 1913, vol. 26. p. 139. 



