
664 - STRATIGRAPHICAL GEOLOGY. [Book VI. 
America, Cheetetes, Thecia, Halysites or chain coral, Syringopora, 
and Tetradium. The rugose corals are likewise abundant, some - 
conspicuous genera being Stawria, Cyathaxonia, Cyathophyllum, 
Zaphrentis, Petraia, Omphyma (Fig. 3827), Strombodes, Ptycho-— 
— phyllum, and Acervularia (Fig. 327). The echinoderms were 
represented by star-fishes (Palwaster, Paleasterina, Palxocoma, 
Lepidaster), brittle-stars (Protaster, Hucladia), many forms of 
crinoids (Actinocrinus, Cyathocrinus, Glyptocrinus, Hucalyptocrinus, 
Taxocrinus, &c.), and particularly by species of the extinct Palzo- 
zoic order of cystideans (Echinospherites, Spheronites, Plewrocystites, 
Hemicosmites). The annelides of the Silurian sea-bottom comprised 
representatives of both the tubicolar and errant orders. To the 
former belong the genera Cornulites, Ortonia, Conchicolites, Serpulites, 
and also the still living genus Spirorbis. ‘The errant forms are 
known only by their burrows or trails which occur in immense 
profusion on the surfaces of shales and sandstones. Names have 
been given to these markings (Arenzcolites, Chondrites, Nereites, 
Scolithus, &c.). | 
The crustacea of the period have been abundantly preserved and 
form some of the most familiar and distinctive fossils of the system. 
Within the last few years undoubted cirripedes have been found in 
the Silurian rocks of Britain, Bohemia, and North America 
(Turrilepas, Anatifopsis). Small ostracods abound in certain 
shales, some of the most frequent genera being Hntomis, Beyrichia, 
Prinitia, Leperditia, Aristozoe, Orozoe, Callizoe. The phyllopods, 
which, as we have seen, made their appearance in Cambrian times, 
continue to occur on scattered horizons, and generally not in great 
numbers, throughout the Silurian rocks; characteristic genera are 
Caryocaris, Peltocaris, Discinocaris, Ceratiocaris, Dictyocaris, Crypto- 
caris, and Aptychopsis. But by far the most prolific order is that of 
the trilobites (Fig. 323), which, beginning in the Cambrian, attained 
its maximum development in the Silurian, waned in the Devonian, 
and became extinct in the Carboniferous period. According to 
the census of Barrande in 1872 there were then 1579 known species. 
A few of the primordial genera continued to live on into Lower 
Silurian times, such as Olenus, Agnostus, and Conocoryphe. But 
many new genera made their appearance and continued to live 
through most of the Silurian period. In the lower division of the 
system, characteristic genera are Asaphus, Amphion, Ampya, 
Barrandia, Cybele, Ogygia, Remoplewrides, and Trinucleus; many 
genera are common to both the lower and upper formations (but 
usually with specific distinctions), such as Acidaspis, Calymene, 
Chewvurus, Encrinurus, Homalonotus, Ilenus, Lichas, and Phacops. 
Towards the top of the system eurypterids make their appearance, 
and continue to occupy a prominent place until the Carboniferous 
period. The Silnrian genera are Plerygotus, Hurypterus, Slimonia, 
Stylonurus, and Hemiaspis. 
‘he polyzoa of Silurian times have been tolerably well preserved, 
