176 THE OLD RED SANDSTONE. 
different from the five varieties of this ichthyc lite which occur 
in the lower formation of the system, but generically the 
same. I very lately enjoyed the pleasure of examining the 
bona fide ichthyolite itself —one of the spec-mens of Dura 
Den, and apparently one of the more entire —in the collec- 
tion of Professor Fleming. Its character as a Pterichthys { 
found very obvious; but neither the Professor nor myself 
was ingenious enough to discover in it any trace of the beetle 
of Dr. Anderson.* 
Is it not interesting to finu this very curious genus in both 
the lowest and highest fossiliferous beds of the system, and 
constituting, like the Trilobite genus of the Silurian group, 
its most characteristic organism ?t The Trilobite has a wide 
geological range, extending from the upper Cambrian rocks 
to the upper Coal Measures. But though the range of the 
genus is wide, that of every individual species of which it 
consists is very limited. ‘The Trilobites of the upper Coal 
Measures differ from those of the Mountain Limestone ; 
* This interesting ichthyolite has since been regarded by Agassiz 
as the representative of a distinct genus, to which he gives the name 
Pamphractus. As exhibited in his restoration, however, it seems to 
differ litt's, if at all, (if I may venture the suggestion,) from a Pter- 
ichthys viewed on the upper side. In Agassiz’s beautiful restoration 
of Pterichthys, and his accompanying prints of the fossils illustrative 
of that genus, it is, with but one doubtful exception, the under side 
of the animal that is presented; and hence a striking difference ap- 
parent between his representations of the two genera, which would 
scarce obtain had the upper, not the under side of Pterichthys been 
exhibited. In verification of this remark, let the reader who has ac- 
cess to the Monographie Poissons Fossiles compare the restoration of 
Pamphractus (Old Red, Tab. VL., fig. 2) with the upper side of Pter- 
tchthys, as figured in this volume, Plate I., fig. 1, making, of corrse, 
the due allowance for a difference of species. 
+t See Note M. 
