that the majority of the seeds described in the following pages are most closely allied, the extinct types possess many distinguishing features that throw light on some at least of the factors concerned in the production of the modern type. In many of the Palaeozoic seeds the nucellus stands free within the integument, to which it is attached only in the chalazal region, in contrast to the lateral union between integument and nucellus in the ovules of recent Cycads. It has been suggested by Oliver[793] that the seed of the Conifer _Torreya_ affords a clue to the interpretation of this difference and that the lower part of the seed in Cycads and _Torreya_ represents a later intercalation in the basal portion of the ovule, the ancient type having a free nucellus in contrast to the nucellus of modern seeds which is free only at the apex. It has been objected[794] that there are no adequate grounds for assuming the addition of an intercalated zone or of the elongation of the ovule that this implies, the more probable view being that the lateral union of nucellus and integument represents congenital fusion in the ancestral type, a fusion comparable with that of the coherent petals of a gamopetalous corolla. In the presence of a pollen-chamber most of the Palaeozoic seeds agree with those of recent Cycads, but in the extinct forms it is usually a more highly developed structure. The name pollen-chamber was given by Brongniart[795] to the pollen-containing cavity in the free region of the nucellus in the petrified seeds from St Étienne in ignorance of the use of the same term by Griffith[796] in a posthumous work published in 1852 for the nucellar chamber in _Cycas_[797]. The genus _Stephanospermum_ (fig. 494, A) illustrates the means by which the pollen-chamber was liberally supplied with water and thus adapted to the requirements of fertilisation by motile gametes. The pollen-chamber and its vascular supply paved the way for siphonogamy, that is the development of a pollen-tube for the more direct transmission of the male sperms. The highly developed mantle of tracheal tissue at the periphery of the nucellus in _Stephanospermum_, represented on a reduced scale by the separate vascular strands of other seeds, may be compared with the tracheal investment to the nucellus in the recent Dicotyledonous genus _Cassytha_[798]. The presence of a nucellar vascular system in several Palaeozoic seeds is a feature in which they differ from those of recent Cycads with the exception of _Bowenia_. The retreat of the vascular supply from the immediate neighbourhood of the pollen-chamber in recent Cycads may, as Oliver points out, be correlated with the evolution of the pollen-tube—the substitution of siphonogamy for zoidiogamy. The diagram reproduced in fig. 492 represents a synthetic type based on such seeds as _Stephanospermum_ and _Cardiocarpus_ which illustrate an arrangement of conducting tissue frequently found in Palaeozoic seeds: