Among the fossils of this period may be remarked a number of Trilobites, which then attained their greatest development. Among others, _Calymene Blumenbachii_ (Fig. 23), some _Cephalopoda_, and _Brachiopoda_, among which last may be named _Pentamerus Knightii_, _Orthis_, &c., and some Corals, as _Halysites catenularius_ (Fig. 26), or the chain coral. [Illustration: Fig. 23.--Calymene Blumenbachii partially rolled up.] The Trilobites, we have already said, were able to coil themselves into a ball, like the wood-louse, doubtless as a means of defence. In Fig. 23, one of these creatures, _Calymene Blumenbachii_, is represented in that form, coiled upon itself. (See also _Illænus Barriensis_, Fig. 25.) Crustaceans of a very strange form, and in no respects resembling the Trilobites, have been met with in the Silurian rocks of England and America--the _Pterygotus_ (Fig. 27) and the _Eurypterus_, (Fig. 24). They are supposed to have been the inhabitants of fresh water. They were called “Seraphim” by the Scotch quarrymen, from the winged form and feather-like ornamentation upon the thoracic appendage, the part most usually met with. Agassiz figured them in his work on the ‘Fossil Fishes of the Old Red Sandstone,’ but, subsequently recognising their crustacean character, removed them from the Class of Fishes, and placed them with the _Pœcilipod Crustacea_. The _Eurypteridæ_ and _Pterygoti_ in England almost exclusively belong to the passage beds--the Downton sandstone and the Upper Ludlow rocks. [Illustration: Fig. 24.--Eurypterus remipes. Natural size.] Among the marine plants which have been found in the rocks corresponding with this sub-period are some species of Algæ, and others belonging to the Lycopodiaceæ, which become still more abundant in the Old Red Sandstone and Carboniferous Periods. Fig. 28 represents some examples of the impressions they have left. The seas were, evidently, abundantly inhabited at the end of the Upper Silurian period, for naturalists have examined nearly 1,500 species belonging to these beds, and the number of British species, classified and arranged for public inspection in our museums cannot be much short of that number. [Illustration: Fig. 25.--Illænus Barriensis.--Dudley, Walsall.]