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The popular collected trilobite Phacops rana is well embedded in literature for over a hundred years. Then in 1990 it was renamed Eldredgeops rana. A lot of collectors did not understand why the name change and I would like to attempt to clarify why the change. The purpose of this post is to point out the differences I have observed between Phacops and Eldredgeops and explain why "rana" is an Eldredgeops and not a Phacops. The literature on phacopid systematics is in a mild state of disarray. Authors have built on the errors of previous authors. There is no good English diagnostic description of Phacops based on the type species of P. latifrons. This has resulted in different English definitions of Phacops and causing much confusion. I'll first start with a review of what are the types and where they come from. Types: 1. Phacops Emmrich (1839) described the genus Phacops based on the species Calymmene latifrons Bronn, 1825 from the Middle Devonian (Eifelian Junkerberg Formation), Gerolstein, Germany. Because the holotype has been lost, it has not been clear what to base the diagnosis of Phacops on over many years. Then Struve (1982) illustrated topotype material but it was Basse (2006) who designated the neotype of Phacops latifrons. Now there is a definitive specimen to base the description of Phacops on. I have been fortunate to have traded for a topotype cephalon of Phacops latifrons Definition of topotype - a specimen of a species collected at the locality at which the original type was obtained 2. Eldredgeops Stewart (1927) described Phacops rana milleri from the Middle Devonian (Givetian Silica Shale), Sylvania, Ohio. Struve (1990) designated Phacops rana milleri the type species of Eldredgeops. I believe the different subspecies of Phacops rana described by Eldredge (1972) are different species and are assigned to Eldredgeops. I will to refer to these different species Eldredgeops as the "rana group" as a way to simplify the naming of all the different species. Observed different characters: I do not know what are the diagnostic generic features of Phacops or Eldredgeops. All I'm doing is listing some of the differences I have observed between these two trilobites to show they are different genera. Pictures of Phacops latifrons and Eldredgeops milleri are below for comparison with numbers pointing to the different features. Pictures of Eldredgeops rana from New York are also included so one can compare the two species of Eldredgeops and see how they differ. Now for the first time a topotype specimen of Phacops latifrons is compared with a topotype specimen of Eldredgeops milleri. There is no place in the literature where this is done. 1) marginulation - a raised ridge along the ventral margin of the cephalon. It is present in the "rana group" and absent in P. latifrons. It has been used by Flick and Struve (1984) as a diagnostic feature for their tribe Geesopini. Note: The value of this feature for the tribe has been questioned. McKellar and Chatterton (2009) state "This feature has never really been evaluated from a phylogenetic standpoint" 2) The post ocular ridge is prominent in P. latifrons and is absent in the "rana group" 3) The palpebral area is smaller in P. latifrons than in the "rana group" 4) The palpebral lobe is smaller in P. latifrons than in the "rana group" 5) The number of eye files in the "rana group" ranges from 15-18. E. milleri has 18 and E. rana has 17. In P. latifrons the number of eye files is 14-15. The topotype specimen has15 files with a maximum number of 5 lenes. 6) The maximum number of lenes in P. latifrons is between 4-5; E. milleri has 8-9; E. crassituberculata has 6 or less; E. rana 6 Note: Both P. latifrons and E. norwoodensis from the Cedar Vally Formation have the same number of files (15) in the eye. One might determine that this would result in the palpebral lobe being the same size but this does not happen. P. latifrons is smaller than E. norwoodensis. So there is some other factor affecting the size of the palpebral lobe. 7) The subocular pad is present in P. latifrons and absent in the "rana group" 8) The glabella is inflated and its front wall varies from vertical to slightly overhanging the anterior border in the "rana group" and is not as inflated in P. latifrons 9) Lateral preoccipital lobe is round in P. latifrons and is rectangular in Eldredgeops. To summarize the differences: Eldredgeops is marginulated, has an inflated glabella, a rectangular lateral preoccipital lobe, the palpebral area and palpebral lobe and larger than P. latifrons, and does not have a post ocular ridge and subocular pad. Phacops latifrons is not marginulated and the glabella is not inflated, has a post ocular ridge and a subocular pad and a round lateral preoccipital lobe. the palpebral area and palpebral lobe are smaller than Eldredgeops. Other observations: These two genera occur in different time periods. Phacops latifrons is in Middle Devonian Eifelian and Eldredgeops milleri is in the Middle Devonian Givetian It appears all the phacopid of North America disappear at the end of the Eifelian and Eldredgeops migrates from the Old World fauna into North America in the Givetian. Eldredgeops does not evolve from any North American phacopid. Eldredgeops is in the Tribe Geesopini and all the genera of this tribe have not been validated. If these genera are reexamined, it is possible that Eldredgeops could become a junior objective synonym of an another genus in the Tribe Geesopini. Hopefully, now collectors will understand the differences between Phacops and Eldredgeops and why the "rana" group is now referred to as Eldredgeops.