Published online by Cambridge University Press: 20 May 2016
The study of specimens of the retiolitid graptolites Stomatograptus sp., and Agastograptus robustus with a complete or nearly complete periderm permits a better understanding of the growth and development of retiolitids. Incremental layers deposited in more or less concentric layers on skeletal lists are synchronized with the deposition of sheetlike fusellar bands between the lists. Lists, therefore, become progressively more robust, while the periderm of fusellar bands remain single-layered. It appears that before the development of new distal thecal lists and concomitant deposition of fusellar bands can begin, the next proximal-most thecae must be essentially complete. Retiolitid graptolite thecal growth thus appears to mirror that of “normal” graptolites.