THE UPPER SILURIAN PERIOD. 129 
species of FPlatyostoma (fig. 72, 2%) 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.—Upper Silurian Gasteropods. a, Platyceras ventricosum, Lower Helder- 
berg, America; 4, Exonephalus discors, Wenlock, Britain; c, Holopella obsoleta, Lud- 
low, Britain; d, Platyschisma helicites, Upper Ludlow, Britain; e, Holopella gracilior, 
Wenlock, Britain; 7, Platyceras multisinuatum, Lower Helderberg, America; g, Holc- 
pea subconica, Lower Helderberg, America; , 2’, Platyostoma Niagarense, Niagara 
_ Group, America. (After Hall, M‘Coy, and Salter.) 
(fig. 72, g), Holopella (fig. 72, ¢), Platyschisma (fig. 72, @), 
Cyclonema, Pleurotomaria, Murchisonia, Trochonema, &c. The 
oceanic Univalves (/Zeferopods) are rep- 
resented mainly by species of Ledlero- 
phon; and the Winged Snails, or Prero- 
pods, can still boast of the gigantic. Zhece 
and Conulari@g, which characterise yet 
older deposits. The commonest genus 
of Preropoda, 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 73.—Tentaculites or- 
: : : natus. Upper Silurian of 
often with finer transverse or longitudi-  Europeand North America. 
nal striz as well; and many beds of the 
Upper Silurian exhibit myriads of such tubes scattered promis- 
cuously over their surfaces. 
