500 MULLER ON THE STRUCTURE OF THE GANOIDS, 
give stability to the attempts to arrange fossil and living fish in 
one system. : 
The cuticular appendages exhibit most remarkable and most 
readily ascertained differences between fossil fish. M. Agassiz 
applied them as the groundwork of his classification of fish gene- 
rally, and formed his sections of the Cycloidei, Ctenoidei, Ga- 
noidei and Placoidei, from them. The scales of living osseous 
fishes are usually imbricate, mere or less rounded, and differ, 
excepting the bony shields, in their minute structure from true 
bones; they do not generally contain the radiating corpuscles of 
bones; their surface usually exhibits concentric, elevated lines, 
which are sometimes irregular. 
The difference between the scales which have an undivided 
margin, or the Cycloid, and the ciliated or Ctenoid, is slight, and 
its systematic application is confined within narrow limits*. 
The scales of the Ganoidei (Ag.) are entirely different. They 
are bony, generally of a rhombic form or quadrangular, sel- 
dom rounded or imbricate, their surface is always smooth and 
coated with a layer of enamel; they are generally arranged in 
oblique rows, those in each row being usually united to one 
another by a prolongation of the hinge. This remarkable kind 
of scales occurs in the existing creation in two genera of fish 
only, which were arranged by Cuvier in his Clupee, viz. in the 
genus Lepisosteus from the Mississippi, and Polypterus from 
Senegal and the Nile. 
Cuvier was the first who perceived the identity of the scales 
of Paleoniscus from the Zechstein with the scales of Lepisosteus 
and Polypterus, and who directed attention to the similarity of 
the long upper tail-piece in Paleoniscus and the Sturgeons to 
the marginal covering of this plate with triangular fulera (or 
ridge-tiles), and to the covering of the anterior margin of the 
back fin with similar laminz in Paleoniscus and Lepisosteus. 
From this resemblance he concludes that the Paleoniscus should 
be arranged either near the Sturgeons or in the vicinity of Le- 
pisosteust. 
The idea of giving up this alternative and combining these 
two categories, does not receive any notice in Cuvier’s writings. 
In his examination of the fish belonging to the genus Dipterus, 
he says distinctly, that they agree with the fish found in the 
* See my Treatise on the Natural Families of Fish, Wiegmann’s Archiv, 1843. 
+ Oss. Foss. Nouv. ed. t. v. 2. 1824, pp. 307, 308. 
