Il8 HISTORICAL PALEONTOLOGY. 



which are divided into several zones, capable of recognition by 

 their fossils, and known by local names (Tentaculite Lime- 

 stone, Water-lime, Lower Pentamerus Limestone, Delthyris 

 Shaly Limestone, and Upper Pentamerus Limestone). As a 

 whole, this series may be regarded as the equivalent of the 

 Ludlow group of Britain, though it is difficult to establish any 

 precise parallelism. The summit of the Lower Helderberg 

 group is constituted by a coarse-grained sandstone (the " Oris- 

 kany Sandstone "), replete with organic remains, which have 

 to a large extent a Silurian /^^/>^. Opinions differ as to w^iether 

 this sandstone is to be regarded as the highest bed of the Upper 

 Silurian or the base of the Devonian. We thus see that in 

 America, as in Britain, no other line than an artificial one can be 

 drawn between the Upper Silurian and the overlying Devonian. 



As regards the life of the Upper Silurian period, we have, as 

 before, a number of so-called "Fucoids," the true vegetable 

 nature of which is in many instances beyond doubt. In addi- 

 tion to these, however, we meet for the first time, in deposits 

 of this age, with the remains of genuine land-plants, though 

 our knowledge of these is still too scanty to enable us to con- 

 struct any detailed picture of the terrestrial vegetation of the 

 period. Some of these remains indicate the existence of the 

 remarkable genus Lepidodendron — a genus which played a part 

 of great importance in the forests of the Devonian and Carbon- 

 iferous periods, and which may be regarded as a gigantic and 

 extinct type of the Club-mosses {Lycopodiace(B). Near the 

 summit of- the Ludlow formation in Britain there have also 

 been found beds charged with numerous small globular bodies, 

 which Dr Hooker has shown to be the seed-vessels or " spor- 

 angia" of Club-mosses. Principal Dawson further states that 

 he has seen in the same formation fragments of wood with the 

 structure of the singular Devonian Conifer known as Proto- 

 taxites. Lastly, the same distinguished observer has described 

 from the Upper Silurian of North America the remains of the 

 singular land-plants belonging to the genus Psilophyton, which 

 will be referred to at greater length hereafter. 



The mari?ie life of the Upper Silurian is in the main con- 

 stituted by types of animals similar to those characterising the 

 Lower Silurian, though for the most part belonging to different 

 species. The Protozoans are represented principally by Stro- 

 matopora and Ischadites, along w^ith a number of undoubted 

 sponges (such as Aniphispongia, Asfrceospongia, Asfylospongia, 

 and Palcsomanofi). 



Amongst the C(jelef iterates, w^e find the old group of Grap- 

 tolites now verging on extinction. Individuals still remain 



