_Mitrospermum compressum_ (Williamson). Mrs Arber[968] proposed the name _Mitrospermum_[969], suggested by the peculiar form of the seed-base, as a substitute for _Cardiocarpon_ for Williamson’s species _C. compressum_[970] from the Lower Coal Measures of Lancashire. The seed is platyspermic and there is some evidence that it split into two valves along the principal plane (the longer axis of the section, fig. 494, L). The diagrammatic and partially restored longitudinal section reproduced in fig. 494, K, shows the main features: a sarcotesta, _sa_, covers the surface of the testa as a thin layer except at the edges of the flattened sides where it forms a wing-like border; preserved as an impression the seed would be assigned to _Samaropsis_. The sclerotesta, _sc_, has a pointed apex which surrounds the lower third of the micropyle and a broad base perforated by the chalazal vascular strand. There was probably a narrow inner flesh as in _Trigonocarpus_ and recent Cycadean seeds (fig. 494, K, _cf_). The nucellus was free from the integument except at the base, as in _Trigonocarpus_ and _Stephanospermum_ (fig. 494, K, _n_): internal to the shrivelled remains of the inner flesh there was a nucellar tapetum surrounding the megaspore. Details as to the pollen-chamber are lacking though there are indications that it resembled that of some species of Cordaitean seeds. The main vascular supply passes through the sclerotesta and then forms a low cushion of short reticulate elements below the base of the nucellus from which two bundles are given off (fig. 494, K, _v_) in the principal plane. The course of the bundles which pierce the sclerotesta led Mrs Arber to remove this seed from _Cardiocarpus_, as recently defined by Bertrand[971], since in that genus the integumental bundles have their origin below the sclerotesta. In the course of the vascular bundles _Mitrospermum_ is intermediate between _Rhabdospermum_ and _Taxospermum_. This genus is founded on a detached seed, but its resemblance to undoubted Cordaitean species favours its attribution to that group though, as Mrs Arber points out, some markedly platyspermic seeds are known to have been borne on fern-like fronds and _Mitrospermum_ may belong to some genus of Pteridosperms. =Diplotesta.= Brongniart.