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 addition 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 construct 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 Carboniferous periods, and which may be regarded as a gigantic and extinct type of the Club-mosses (Lycopodiaceoe).  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 “sporangia” 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 Prototaxites.  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 marine life of the Upper Silurian is in the main constituted 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 Stromatopora and Ischadites, along with a number of undoubted sponges (such as Amphispongia, Astroeospongia, Astylospongia, and Paloeomanon).

Amongst the Coelenterates, we find the old group of Graptolites now verging on extinction.  Individuals still remain numerous, but the variety of generic and specific types has now become greatly reduced.  All the branching and complex forms of the Arenig, the twin-Graptolites and Dicranograpti of the Llandeilo, and the double-celled Diplograpti and Climacograpti of the Bala group, have now disappeared.  In their place we have the singular Retiolites, with its curiously-reticulated skeleton; and several species of the single-celled genus Monograptus, of which a characteristic species (M.  Priodon) is here figured.  If we remove from this group the plant-like Dictyonemoe, which are still present, and which survive into the Devonian, no known species of Graptolite has hitherto been detected in strata higher in geological position than the Ludlow.  This, therefore, presents us with the first instance we have as yet met with of the total disappearance and extinction of a great and important series of organic forms.

[Illustration:  Fig. 58.—­A, Monograptus priodon, slightly enlarged.  B, Fragment of the same viewed from behind.  C, Fragment of the same viewed in front, showing the mouths of the cellules.  D, Cross-section of the same.  From the Wenlock Group (Coniston Flags of the North of England). (Original.)]