Article: Apical development in turritellid classification with a description of Cristispira pugetensis gen. et sp. nov.
Richard C. Allison
The nature of primary spiral ribs on apical whorls of Turritella (s.l.) is utilized for subdivision of the genus. However, variation causes some doubt as to the actual value of this criterion. Recognized variation is of two kinds: (1) 'variation' within a stock or group arising from improper grouping of species due to inadequate descriptive notation, and (2) actual variation displayed by various specimens of single species. Inadequate application of the terms 'unicarinate', 'mesocostate', 'unicostate', 'bicarinate', 'bicostate', &c. has confused efforts to arrange six Gulf Coast Eocene taxa in homogeneous groups. Notational systems proposed by Marwick (1957a, b) and Kotaka (1959) permit uniform description of apical developmental characters, and provide a firm base for taxonomic differentiation. The six taxa discussed belong to three separate apical types.Some species are constant in apical development, while others show considerable variation. A given spiral rib may appear at various distances from the apex, but always seems to appear in a constant order relative to other spiral ribs. As long as this sequential order of appearance remains constant, Kotaka's notation allows continued use of apical ontogenetic features for taxonomy. This constancy of sequential appearance of primary spirals is not yet demonstrated for all apically variable turritellas; further investigation is needed to confirm apical developmental characters as reliable tools for all turritellids.A new turritellid from the Eocene of Washington, Cristispira pugetensis gen. et sp. nov., illustrating orderly apical variation, is described and represents a new stock seemingly unrelated to presently known species from the north Pacific.