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Eunotia is the largest and most diverse genus within the family Eunotiaceae, a primarily freshwater group of diatoms often found in dilute, acidic and humic-stained environments. Species in this genus are characterized by being asymmetric along their apical axis, symmetric about the transapical axis, and with a simple and reduced raphe system situated largely on the mantle and restricted to the apical ends of the valve. In addition, Eunotia taxa have one or more rimoportula per valve, usually close to the apex. Because of their reduced raphe system, coupled with the presence of rimoportulae, Eunotia and its relatives are often viewed as the oldest lineage of raphe-bearing diatoms. To date, the oldest remains of Eunotia species have been reported from the early to middle Eocene, including from the Giraffe Pipe locality, an ancient Eocene fossil site located in northern Canada near the Arctic Circle. Rocks from this site contain a large and diverse assemblage of Eunotia taxa. The purpose of this study is to begin to characterize this assemblage with descriptions of three new species, Eunotia giraffensis sp. nov., E. petasum sp. nov. and E. pseudonaegelii sp. nov. The new species, representing the longest specimens found at the Giraffe Pipe locality, each possess characteristics common to Eunotia making them easily assigned to this genus. Because the Eunotia lineage was well established by the early part of the Eocene, it is likely to be significantly older. 

When diatoms undergo vegetative cell division the new siliceous wall components are slightly smaller than those of the parent because they are produced within the confines of the parent wall. Thus, with continued growth the mean size of cells in a population declines. Given this unique feature of diatom cell division, if the growth of a species in a lake increases (decreases) under more (less) favorable conditions, then the mean size of the resulting population will decline (increase). Numerous paleolimnological investigations rely on shifts in the relative abundances of diatom species over time to infer lake conditions. Although relative abundance data yield information about the dominance of species in the community, they do not necessarily provide evidence about growth of a given species. For instance, a species could have increased in growth, but simply to a lesser extent than other taxa, resulting in a decline in relative abundance. In a similar fashion, relative abundance values can be misleading when used to infer environmental change, such as trophic status change in lakes. We propose that including data on mean size of diatom valves can yield greater insight into changes in growth and improve observations and conclusions based on relative abundance data. To test this concept, we examined changes in the mean diameter of Aulacoseira ambigua (Grunow) Simonsen valves relative to known shifts in lake trophic status in a core from Bantam Lake, Connecticut, representing * 130 years of sediment accumulation. The mean valve diameter of A. ambigua declined from 9.7 to 7.6 lm, with the largest declines clearly tracking significant increases in trophic status. We conclude that changes in the mean size of diatom frustules over time can provide valuable information for understanding long-term environmental changes.