Polydiadema bonei (Woodward, 1856)
This species occurs in the Grey Chalk and is common in the sandy facies of the SW. The superficially similar Gautheria (e.g. Gautheria spatulifera) and Phymosoma (e.g. Phymosoma koenigii) occur in the White Chalk, and from a practical point of view can be distinguished from Allomma on that basis. The contemporary Tetragramma can be distinguished by its biserial pore pairs (compare). There is some ongoing confusion regarding the taxonomic assignment of this and other related species, relating to changing opinion as to whether these species belong to the genus Polydiadema, Allomma or another.
1). Adoral view of a very well preserved individual, retaining semi articulated primary spines (x2.1, Grey Chalk, in the collections of the Booth Museum, BMB 002546, by kind permission of John Cooper).
2). (A) Adapical view of an isolated test from the sandy chalk of Wilmington, Devon (x5.5, Randell Collection, RR 0676). (B) Adoral view of the above specimen (Fig. 1)(x3.6).
3). Low angle lateral (side) view of the above specimen (Fig. 2(A)).
4). Detail of Fig. 1, showing the primary spines and elements of the lantern (top-left)(x6.4).