l8 9 I -l Fossil plants of the Coal-measures, 175 



lar laminae, the inner extremities of which, though commenc- 

 ing their growth at different periods of life, all start from the 

 medullary border of the vascular zone and extend to the pe- 

 riphery. Among the numerous specimens examined the num- 

 ber of laminae varied from 44 in a small specimen to 11 20 

 similarly arranged laminae in a larger one. The transition 

 between the petiolar and trunk structures was not only ob- 

 served in all stages from many fragments, but specimens 

 were studied in which stem and petiole are organically united, 

 demonstrating not merely that Lyginodendron Oldhamium is a 

 true fern, probably belonging to the Sphenopterids, but also 

 that the stems of some, at least, of the Carboniferous ferns 

 " developed their xylem or vascular structure exogenously 

 through the instrumentality of a meristemic zone of the in- 

 nermost cortex, which practically must be regarded as a 

 cambium layer." 



Additional observations on the growth of Heterangium 

 Gmevii are contributed, without establishing its true affinities, 

 though it seems likely eventually to prove to be a fern. The 

 discovery of the vegetative organs of Bowmanites ( Volkmannia) 

 Dazvsoni and the study of their structure shows an organiza- 

 tion generically identical with that of the Asterophyllites de- 

 scribed by the author in a former memoir, and the plant 

 described by Renault as Sphenophyllum Siephanense. The trian- 

 gular central vascular axis is the most conspicuous character 

 in the three types. Bowmanites, though it can be regarded 

 as generically identical with the Sphenophylla whose fruits 

 have been definitely correlated, is most strongly allied to the 

 Sphenophylloid type. Williamson calls attention to the fact 

 that evidence is being obtained of the existence of Carboni- 

 ferous plants whose branches bore both Asterophyllitean and 

 Sphenophylloid leaves (a view supported by Stur's researches 

 among the Schatzlar Calamariae), and that Sphenophyllum and 

 some forms of Asterophyllites should be united in the same 

 genus. All these types, however, he regards as belonging to 

 the great family Calamari^, of which Equisetum is "a poor, 

 feeble and degraded member," rather than the central type. 



Washington, D. C. 



