The seeds included in this group are for the most part Radiosperms, but in its slightly developed bilateral symmetry _Conostoma oblongum_ is a type transitional between Radiosperms and Platysperms. The testa may be ribbed and the ribs vary in number. The nucellus (megasporangium) is united to the integument not only at the base but laterally as far as the shoulder of the seed up to a level corresponding to the base of the pollen-chamber (lagenostome) as in all recent Cycads and in the majority of Conifers. The seeds proper apart from the cupule are supplied with a single set of vascular bundles; there is no vascular tissue in the nucellus, a feature no doubt correlated with the fusion of nucellus and integument[818]. The free portion of the integument is more or less deeply lobed or, in _Lagenostoma_, it forms a pyramidal canopy of fused lobes enclosing the lagenostome. The presence of a tapetal zone surrounding the megaspore is believed to be a feature characteristic of the group[819]. The testa, wholly or partially ribbed, is relatively thinner than in the Trigonocarpales and Cardiocarpales, and in its differentiation agrees less closely with the testa of recent Cycadean seeds. In _Lagenostoma_ and possibly in other genera a loose sheath or cupule surrounded the ovule, while in _Gnetopsis_ a similar envelope enclosed two to four seeds. The microspores are multicellular and smaller than those of Trigonocarpales, the average dimensions (_Conostoma_, _Physostoma_, _Lagenostoma_) being 67μ × 52μ. Genera. _Physostoma_; _Conostoma_; _Sphaerostoma_; _Lagenostoma_; _Gnetopsis_. _Lagenostoma_ may safely be referred to _Lyginopteris_, and _Sphaerostoma_ with but little risk of error to _Heterangium_: the parent-plants of the other genera are unknown, but all may be regarded as the seeds of Pteridosperms and probably of genera more nearly allied to the Lyginopterideae than to the Medulloseae. The genus _Lagenospermum_, recently instituted by Nathorst[820], is dealt with in Chapter +xxxi.+ =Physostoma.= Williamson. _Physostoma elegans_ Williamson.