4 
PartIL Secr. ii §1.] SILURIAN, 663 
- Silurian period had a cryptogamic vegetation in which lycopods and 
_ ferns no doubt played the chief part. 
In the fauna of the Silurian rocks the most lowly organisms 
known are foraminifera, of which several genera, including the still 
living genus Saccammina, have been detected. Among these forms 
may perhaps be included, the singular fossils described as Ischadztes, 
Rieceptaculites, and Nidulites, of which the true relations are not yet 
well understood. ‘The Silurian seas possessed representatives of the 
calcareous and of the siliceous sponges of modern times. Under the 
former group may be placed the genus Archzocyathus of the Lower, 
and the genera Astrzospongia and Amphispongia of the Upper 
Silurian rocks; under the latter group come Astylospongia and 
Protachillewm. With these fossils may be placed the abundant and 
still doubtful form Stromatopora. 

Fic, 322.—Grour oF Lower SILURIAN GRAPTOLITES. 
a, Monograptus (Graptolithus) priodon (Bronn); b, Phyllograptus typus (Hall); ¢, Dip- 
lograptus folium (His.); d, Rastrites peregrinus (Barr); e, Didymograptus Mur- 
chisonii (Beck); /, Monograptus (Graptolithus) Sedgwickii (Portl.); g, Dicrano- 
graptus ramosus (Hall); h, Tetragraptus Hicksii (Hopk.). 
Some of the most plentiful and characteristic denizens of the 
Silurian seas were undoubtedly the various hydrozoan genera united 
under the common name of graptolites (Fig. 322). Among the 
monoprionidian forms, or those with a single row of cells, the 
genera Rastrites and Monograptus (Graptolithus) are abundant. The 
diprionidian forms, or those with two rows of cells, specially cha- 
racteristic of the lower subdivision of the Silurian system, are richest 
in genera, of which some of the commonest are Diplograptus, Dicello- 
graptus, Didymograptus, and Climacograptus. 
Corals must have swarmed on those parts of the Silurian sea-floor 
on which calcareous accumulations gathered, for their remains are 
abundant among the limestones, particularly in the upper division of 
the system. Among the tabulate forms are the genera Favosites, so 
characteristic in the Upper Silurian limestones of Europe and 
