Article: The hystero-ontogeny of Lonsdaleia McCoy and Thysanophyllum orientale Thomson

Increase in thirteen specimens of Lonsdaleia and two of Thysanophyllum orientale from Great Britain was studied by means of closely spaced acetate peel sections. With the exception of one example of intermural increase, only lateral increase was observed. Lonsdaleia is characterized during early development by having a long cardinal septum, the axial end of which forms the median lamella of the axial structure. Septal insertion is at first of the zaphrentid mode but subsequently becomes variable, depending on the development of minor septa. Most daughter corallites are radially orientated to the parent, but some are tangential. Interseptal dissepiments are typically formed first, followed by their modification to lonsdaleoid dissepiments.Development in Thysanophyllum orientale, type species of Thysanophyllum, resembles that in Lonsdaleia in the nature of the cardinal septum and dissepimental development. These characters, together with the presence in a few adult corallites of T. orientale of a weakly developed axial structure suggest that Thysanophyllum is related to the Lonsdaleiidae rather than the Lithostrotionidae as has been suggested by some workers.