A description of the morphological features of _Trigonocarpus Parkinsoni_ and _T. shorensis_ is given in the chapter on _Medullosa_ (p. 117), as there is good evidence that they are the seeds of that genus. There is considerable difference in size and to some extent in the form of casts included in _Trigonocarpus_ and, in the absence of anatomical data, it is hardly possible to determine the actual systematic position of many of the specimens so named. Dr Arber[850] has recently proposed a new generic name _Schizospermum_ for casts very like those of _T. Parkinsoni_, but distinguished by the splitting of the shell into three valves, a character which leads him to conclude that it is the external surface which is preserved and not a mere cast of the seed-cavity. It is, however, more probable that the specimens are casts of a split sclerotesta. In _Trigonocarpus pusillus_[851] the shell is divided into three valves, the dividing lines being marked by greatly reduced ribs, and in _T. schizocarpoides_ Grand’Eury[852], a species that may not be a true _Trigonocarpus_, there is also evidence of splitting. Arber points out that the species _Rhabdocarpus Boschianus_ Berg. is founded on a _Trigonocarpus_ from which the outer flesh has disappeared leaving the shell as the external covering. _Trigonocarpus_ seeds are widely distributed in Carboniferous and Permian rocks in Europe and North America: from the latter continent Newberry[853] has described several different forms that afford good examples of the abundance and variety of the genus. Some of the specimens included by Newberry[854] in _Trigonocarpus_ are probably distinct generic types: his species _T. multicarinatus_ may be identical with the ribbed cast shown in fig. 506, A. The casts described by Lindley and Hutton and by other authors as _T. Dawesi_[855] are nearly 5 cm. long, and if these are correctly included in the genus they point to the occurrence of seeds much larger than the type-species. The French species _Trigonocarpus pusillus_[856] Brongn., one of the smallest Palaeozoic seeds, from 6·5 to 15 mm. long, differs from _Trigonocarpus Parkinsoni_ and _T. shorensis_ in the absence of prominent ribs and in the much feebler development of the sarcotesta. Specimens of the German type _T. sporites_ Weiss, believed by some authors to be megaspores, were described by Zeiller[857] from Valenciennes as seeds: these are from 2·5 to 3·5 mm. long and have three small ribs. Zeiller quotes the presence of cell-outlines on the surface as evidence of their seed-nature, but it may be that this feature represents a sculpturing of the exine of a spore. Typical _Trigonocarpus_ seeds agree in several morphological characters with those of recent Cycads. They differ in the lack of a lateral union between nucellus and integument; the presence of nucellar tracheids, though a feature shared with _Bowenia_, distinguishes them from the majority of recent Cycadean seeds. In the comparatively long and fleshy