DESCRIPTIVE REMARKS. lvl 
range being from Upper Llandovery to Middle Devonian strata.* The 
characteristic examples are P. anglicus, and P. gigas. 
Of the genus Eurypterus (according to the same authority), about 
twenty-two species are known, eleven of them in British strata, the 
Geological range of this genus being from the uppermost Silurian 
(Downton Sandstone and Passage Beds) to the lowest Carboniferous 
strata. 
Other Crustacea of this order belonging to the genus Stylonurus, 
including six species, of which S. Symondsw (formerly called Euryp- 
terus), figured on Plate xxxil., fig. 8, is an example, reduced to one- 
third of natural size, from ‘‘ Cornstones”’ of the Old Red Sandstone, 
Rowlestone, in Brecknockshire; the Geological range of the genus 
being from Uppermost Ludlow to the Lower Devonian of Forfarshire. 
Several forms of Crustacea allied to some of those just mentioned 
have also been collected from Upper Old Red Sandstone at Kiltorcan.t+ 
The true Old Red Sandstone period may correctly be termed the 
age of fish, their remains being the predominant fossils; they prin- 
cipally belong to the great order of Ganoids, a few only to Placoids (to 
which division the recent Sharks and Rays belong), as these orders are 
defined by the late Professor Agassiz, whose magnificent and extensive 
works on Fossil Fish have so largely benefited science. 
The fossil fish of the Paleozoic Rocks all belonged to the division 
with heterocercal tails, being unsymmetrical, with a prolongation of 
the vertebral column into the upper lobe, as in the Permian genus 
Pygopterus (Fig. 15 a), and in the recent Lepidosteus, Sharks, Rays, 
and Sturgeons; whilst the majority of living fish have the homocercal 
or symmetrical form of tail in which the vertebral column terminates 
Pigs lo: 
Heterocercal tail. Homocercal tail. 
Reduced one-third. 
Pygopterus ( fossil). Clupea, Herring (recent). 
* Monograph by H. Woodward, F. G. S., &c., in Palewontographical Society, 
Vol. xix., 1866, p. 19. 4 
+ British Association Reports, 1868 and 1869. 
