206 
SILURIA. 
[Chap. IX. 
also Lower Silurian forms (P. strangulata, Salter, P. simplex, Jones, &c), and 
Upper Silurian (P. umbilicata, Jones, P. Roemeriana, Jones, &c.). The genus 
Leperditia (composed of larger, smooth forms of this group) also furnishes some 
British Silurian species * j Entomis, another Phyllopod, is found both in Scot- 
land (Pentland Hills) and Shropshire; and two or three large fossils of the 
Phyllopod group have also been collected in the black slates of the South of 
Scotland (p. 152). 
No other tribes of Crustaceans than the Trilobites and the Phyllopods above 
noted, with the Phyllopodous Hymenocaris vermicauda, mentioned at p. 44, 
have been found in the Lower Silurian rocks of Britain. In the sequel it 'will 
be seen, that a group of Crustacea, vastly superior in size, but perhaps not of 
higher organization than those above cited, is found in the uppermost zone of 
the system. 
Fossils (48). Lower Silurian Trilobites. 
1. Cheirurus clavi- 
frons, Dalm. 2. Cy- 
bele verrucosa,Dalm. 
3. Phacops conoph- 
thalmus, Beck. 4. 
Harpes Flanagani, 
Portl. 5. Remopleu- 
rides dorso-spinifer, 
With regard to the distribution of the British Trilobites, I would direct 
the attention of the reader to the Descriptions of the Lower Palaeozoic 
Fossils in the Cambridge Woodwardian Museum, by Professor M'Coy, in order 
that he may see how many species, even in that one rich collection, are common 
to what have been so long and so generally called the Lower and Upper Silurian 
rocks. 
He will see that out of fifty-six species of Trilobites there enumerated 
(being double the list first published in my original work) nearly all the new 
forms have been found in localities of Montgomery, Radnor, Brecon, Carmar- 
then and Pembroke, which were laid down by me as Lower Silurian on the 
map, — a strong indication that the region first described as Silurian still affords 
the best fossil types. 
It is here important to remark that the numerous Footmarks in the 
Potsdam Sandstone or lowest Silurian rock of North America, which were 
at first supposed to have been made by Tortoises, were, in consequence of 
the discovery of better specimens, subsequently referred by Professor 
Owen to Crustaceans f ; and tracks of a similar kind, but far smaller, have 
been found in our own country (see above, p. 151). Hence this last class 
of animals may still be considered the highest type of life in the earliest 
* Jones, Ann. & Mag. Nat. Hist. 1856, vol. xvii. specimens and casts of a very great number of 
p. 95. these large and curious impressions, cannot be boo 
t See Quart. Journ. Geol. Soc.vol.vii. p. 247, and much commended. In default of these labours, 
vol. viii. p. 223. The zeal of that acute and La- most erroneous ideas would have been propagated 
borious geologist Sir William Logan, in procuring respecting the Lower Silurian fauna. 
Portl. 6. Acidaspis 
bispinosa, M'Coy. 7. 
Am pyx nudus, Mur. 
8. Cyphoniscus soci- 
alis, Salter : the line 
beneath indicates its 
natural size. 
