THE UPPER SILURIAN PERIOD. 



131 



here, and the group has with reason been regarded as allied 

 to the Violet-snails (lanthina) of the open Atlantic. The 



Fig. 72. Upper Silurian Gasteropoda, a, Platyceraa ventricoaum, Lower Hel- 

 derberg, America ; b, Euomphalua diacora, Wenlock, Britain ; c, Holopella obaoleta, 

 Ludlow, Britain ; d, Platyscfiisma helicites, Upper Ludlow, Britain ; e, Holopelia 

 gracilior, Wenlock, Britain ; /, Platyceraa multisinuatum, Lower Helderberg, 

 America ; g, Holnpea aubconica. Lower Helderberg, America ; h, h' Platyoatoma 

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



species of Platyostoma (fig. 72, h} also belong to the same 

 family ; and the entire group is continued throughout the 

 Devonian into the Carboniferous. Amongst other well-known 

 Upper Silurian Gasteropods are species of the genera Holopea 

 (fig. 72, g), Holopella (fig. 72, e}, Platyschisma (fig. 72, d), 

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

 oceanic Univalves (Heteropods) are rep- 

 resented mainly by species of Bellero- 

 phon; and the Winged Snails, or Ptero- 

 pods, can still boast of the gigantic Theca 

 and Conularice, which characterize yet 

 older deposits. The commonest genus 

 of Pteropo&a, however, is Tentaculites (fig. 

 73), which clearly belongs here, though 

 it has commonly been regarded as the 

 tube of an Annelide. The shell in this 

 group is a conical tube, usually adorned 



with prominent transverse rings, and Fig. 13. Tentacuiite* 



. . - ,. ornatua. Upper Silurian of 



often with finer transverse or longitudi- Europe and North America. 



nal striae as well ; and many beds of the 



Upper Silurian exhibit myriads of such tubes scattered promis- 

 cuously over their surfaces. 



