LYGINODENDRON 4°i 



of a Fern, constitutes the pollen-bearing organs of this 

 genus of Pteridosperms, a fact that must affect our 

 judgment on many other Palaeozoic fructifications. In 

 bearing its microsporangia on a pinna of the same frond 

 which in other parts has purely vegetative characters, 

 Lyginodendron shows a lower degree of differentiation 

 than any Seed-plant previously known. The reproduc- 

 tive organs of this genus present the same combination 

 of characters which we found in the vegetative structure. 

 While the highly organised seed indicates a clear affinity 

 with Cycadophyta, the microsporangiate organs are 

 of a more Fern-like nature. 



Lyginodendron oldhamium has been described at 

 considerable length because, from our exceptionally 

 complete knowledge of the plant in all its parts, it may 

 best serve as the type of the Pteridospermeae. We 

 now pass on to review, more briefly, other representatives 

 of the class. 



II. Heterangium 



Several species of this genus have been described, 

 some from our own country, others from the Carboni- 

 ferous strata of France and Germany. The geological 

 range of these plants is considerable, for some species 

 have been found in the Calciferous Sandstone rocks of 

 Scotland, at the very base of the Carboniferous forma- 

 tion, while others appear in the Coal-measures, and 

 others again in the Permian beds of Autun, in France. 

 Our description will be based, in the first instance, on 

 the Heterangium Grievii of Williamson, the oldest of 

 the species at present described, and the one which 



