Abstract: Through the application of some statistical analyses to the abundant Icriodus specimens,Lower Devonian conodonts, collected from the Monitor and Toquima ranges, Central Nevada, U.S.A.,the evolutionary change in this fossil group is investigated. Under the light of ontogenetic concepts andobservations, the statistical frequency distributions of «elongation index» in the populations, which ismodified from width-length ratio as I-W/L, are evaluated mathematically. By the interpretations concerning evolution and taxonomy, the two species. I. steinachensis and I. claudiae are redescribed andthe hypodigms of the two are given. Mathematical methods, graphs, and interpretations are explainedin detail.