342 
SILURIA. 
[Chap. XIII. 
same there as in the eastern hemisphere, but in several cases the species are 
identical with those found in the Magnesian Limestone and Zechstein. These 
fossils have been described by Messrs. Meek and Hayden*, Swallow and Hawnf, 
Shumard J, and very recently by Geinitz §. 
On the whole, therefore, if only a very few species of Mollusks are common 
to the older rocks and the Permian, the latter still retains its connexion with 
the former through the genera Bellerophon, Edmondia, Athyris, Chonetes, Pro- 
ductus, Spirifer, and Strophalosia, its peculiar group of Corals, and of Polyzoa 
(Fenestellse &c). It is the constant diffusion of many individuals of such forms 
which induced my associates and myself to hold firm to the term ' Permian,' as 
marking the close of primeval life, and in separating it entirely from the Trias 
and other Secondary deposits, in common with which it contains no animal 
form. 
The fossil Fishes of this era, of which more than forty species are known, all 
belong, like their precursors in more ancient times, to the division with hetero- 
cercal tails, — a distinction that becomes evanescent in the succeeding Secondary 
and Tertiary formations, and in our era is chiefly confined to the Lepidostean, 
Ray, Shark, and Sturgeon families, nearly all other species now living (about 
8000 in number) having homocercal tails. 
Of true heterocercal Fishes of the Permian group, two figures only are here 
given, by which the reader will see that the back-bone is prolonged with a bend 
into the end of the upper lobe of the tail ; whilst in the homocercal fishes, or 
those common in the younger rocks and in the present day, it terminates in the 
middle, between the equally balanced fins of the tail. 
Palseoniscus, Pygopterus, Ccelacanthus, Pleuracanthus (Xenacanthus), Acan- 
thodes, Acrolepis, and Platysomus, which are the prevailing genera of the 
Permian era (the same forms being found in England, Germany, and Russia), 
are also known in the Carboniferous epoch; but all the Permian species are 
distinct. 
These Fishes are chiefly found in the Kupfer-Schiefer underlying the Zech- 
stein in Germany ; and in England they are met with in the equivalent deposit, 
the Marl-slate. Some small Palseonisci and a species of Acrolepis have lately 
been described from the upper beds of the Magnesian Limestone in Durham l|. 
Lastly, in enumerating the chief features of the Permian fauna, we have to 
* Trans. Albany Instit. 1858. || Kirltby, ' On some remains of Fish and Plants 
t Trans. Acad. Sci. St. Louis, 1858. from the Upper Limestone, &c.,' Quart. Journ. 
I Trans. Acad. Sci. St. Louis, vol. i. p. 292, 1858. Geol. Soc. vol. xx. p. 345, pi. 18. 
I Carbonformation und Dyas. Dresden, 1866. 
Fossils (86). Permian Fish. 
Platysomus striatals, Agassiz ; from the Marl-slate, Durham. 
