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, Platycerax ventricosum, Lower Hel- 

 derberg, America ; 6, Euomphalus discors, Wenlock, Britain ; c, Holopella obaoleta, 

 Ludlow, Britain ; d, Platyschisma lielicites, Upper Ludlow, Britain ; e, ffolopelia 

 gruc.il i nr, Wenlock, Britain ; /, Platyceras multisinuatum, Lower Helderberg, 

 America ; g, Holopea subconica, Lower Helderberg, America ; h, h' Platyostama 

 Niagarense, 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, <?), Platyschisma (fig. 72, rf), 

 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 Thecce 

 and Conularia, which characterize 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 Annelide. The shell in this 

 group is a conical tube, usually adorned 



with prominent transverse rings, and ri - '3- Tentacuiitea 



. , .. ornatug. 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. 



