Parr Il. Scr. ii §1.] SILURIAN. 665 
- 
and present many peculiarities of structure. One of the most 
abundant genera is Fenestella, which ranges from Lower Silurian to 
Permian rocks; another, Péclodictya, ascends into the Carboniferous 
system. Other genera are Retepora, Paleschara, and Hippothoa. So 
abundant are the brachiopods and so characteristic on the whole are 
the species of them occurring in certain Silurian zones or bands, that 
these fossils must be regarded as of special value for purposes of 
stratigraphical comparison. The old and still living genera Discina, 
Lingula, and Crania are found on different horizons in the Silurian 
series. Characteristic types are Acrotreta, Atrypa, Leptena, Meri- 
stella, Orthis, Pentamerus, Porambonites, Rhynchonella, Siphonotreta, 
_Spirifera, Stricklandinia, Strophomena, and Triplesia. Some of these 





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Fic. 323.—Grovur or Lower SILuRIAN TRILOBITES. 
1, Iilenus Davisii (Salt.) G); 2, Calymene brevicapitata (Portl.); 3, Ogygia Buchii 
(Brongn.) (4); 4, Asaphus tyrannus (Murch.) (4); 5, Ampyx nudus (Murch.) (3); 
6, Aiglina binodosa (Salt.); 7, Acidaspis Jamesii (Salt.); 8, Trinucleus Lloydii 
(Murch.). 
are particularly distinctive of certain zones. Thus the Pentamerz 
are so common in the so-called middle Silurian rocks in Britain that 
these strata received the name of the “ Pentamerus beds” (Fig. 326). 
Orthis is most abundant in species in the lower part of the Silurian 
system: Lhynchonella and Spirifera occur chiefly in the upper. The 
lamellibranchs have been less abundantly preserved ; some of their 
“most frequent genera are the monomyarian Ambonychia (Fig. 323) 
and Ptermmea and the dimyarian Ctenodonta, Modiolopsis, Goniophora, 
Orthonota (Fig. 328), Clecdophorus (Fig. 325), Palearca, and Redonia 
Cie. 324). 
Of the gasteropods of the Silurian seas upwards of 1500 species 
have been named; some of the more frequent genera are Acroculia, 
