Williamson, W. C. (1880). On the Organization of the Fossil Plants of the Coal-Measures. Part X. Including an Examination of the Supposed Radiolarians of the Carboniferous Rocks. Philosophical Transactions of the Royal Society of London. 171(0): 493-539., available online at
https://archive.org/details/philtrans04248024 page(s): pp. 523-524 pl. 20 fig. 81