THE UPPER SILURIAN PERIOD. 



129 



species of Platyostoma (fig. 72, ti) also belong to the same 

 family ; and the entire gtoup is continued throughout the 

 Devonian into the Carboniferous. Amongst other well-known 

 Upper Silurian Gasteropods are species of the genera Holopea 



h> 



Fig. 72. Upper Silurian Gasteropods. a, Platyceras ventricosum, Lower Helder- 

 berg, America ; b, Euomphalies discors, Wenlock, Britain ; c , Holopella obsoleta, Lud- 

 low, Britain; d, Platyschisma helicites, Upper Ludlow, Britain ; e, Holopella grac Hi or, 

 Wenlock, Britain ;f, Platyceras multisinuatum, Lower Helderberg, America; g, Holo- 

 pza s-itbconica. Lower Helderberg, America ; h, h' ', Platyostoma Niagarense, Niagara 

 Group, America. (After Hall, M'Coy, and Salter.) 



(fig. 72, g), Holopella (fig. 72, e), Platyschisma (fig. 72, //), 

 Cyclonema, Pleurotomaria, Murchisonia, Trochomim, &c. The 

 oceanic Univalves (Heteropods) are rep- 

 resented mainly by species of Bellero- 

 phon ; and the Winged Snails, or Plero- 

 pods, can still boast of the gigantic Theccc 

 and Conularicz, which characterise yet 

 older deposits. The commonest genus 

 of Pteropoda, however, is Tentaculites (fig. 

 73), which clearly belongs here, though 

 it has commonly been regarded as the 

 tube of an Ann elide. The shell in this 

 group is a conical tube, usually adorned 

 with prominent transverse rings, and 

 often with finer transverse or longitudi- 

 nal striae as well ; and many beds of the 

 IJpper Silurian exhibit myriads of such tubes scattered promis- 

 cuously over their surfaces. 



*'ig 73- Tentaculites or- 

 natu*. Upper Silurian of 

 Europe and North America. 



