718 HISTORICAL GEOLOGY, 



Plants. 



The line of succession for Paleozoic terrestrial plants has been made 

 apparently clear by the observation that the Ehizocarps, the simple and small, 

 mud-growing Acrogens, few in existing species, of which Salvinia and Mar- 

 silea are two of the four modern genera, were the probable source of the 

 spores that so greatly abound in Devonian shales (Dawson). Through the 

 Protosalvinia, according to this author, the line leads up to the Equiseta, 

 that is, to the Calamites and Annularitje of the earliest terrestrial flora. 

 Another simple type of Cryptogam, related to the former in fructification, 

 that of Selaginella, which is represented now by only one single genus and 

 thus shows that it is a type of the past now dwindled, is regarded as the 

 probable source of the Lepidodendrids, and through them of the Sigillarids, 

 or semi-exogenous Acrogens, and of the Yews and other true Gymnosperms. 



The special type among these simpler Cryptogams that was precursor to 

 the Ferns has not been ascertained. Since circinate vernation characterizes 

 both Cycads and Perns, and since a genus of Cycads, Stangeria, now exists 

 in which the foliage is Fern-like, it is probable that the line to the Ferns led 

 beyond to the Cycads, the other grand division of the Gymnosperms, and, 

 therefore, that the Gymnosperms had a double source. 



In the Lepidodendrids, Sigillarids, and related species, Cryptogams reached 

 their culmination, or their greatest expansion in number of species, and their 

 highest perfection in type of structure. The Lepidodendrids have no species 

 in the Permian period, and the Sigillarids none after it. Further, the 

 Equiseta passed, through the Calamodendra, their time of maximum devel- 

 opment during the Carboniferous period. The genus Calamites had later 

 species, but they were smaller, and the associated Equiseta were of the 

 inferior modern type. 



The Cycads culminated in later time ; and the same is true also of the 

 more typical Gymnosperms — the Conifers. 



Invertebrates. 



1. Hydrozoans ; Actinozoans. — The Graptolites, Cambrian in their 

 beginning and Lower Silurian in cidmination, disappear with the Xiower 

 Devonian. The CyatJiophylloid Corals, or Tetracoralla, also dating from the 

 Cambrian, increase in number of genera and species in the Silurian ; with 

 other Corals make coral reefs in the Upper Silurian ; are in much greater 

 numbers, and of larger size, and make still more extensive reefs, but undergo 

 little increase in genera, in the Lower Devonian ; then in the Lower Car- 

 boniferous they almost disappear. Three of the species observed pertain 

 to the three older genera, Cyathophyllum, Zaplirentis, and Phillipsastrea, 

 and three are new genera, Lithostrotion, Cyathaxonia, and Lonsdalia. The 

 recent discoveries of the "Challenger" Expedition report a living species of 

 a Cyathophylloid Coral from the bottom of the ocean. 



