690 MR, T. W. BRIDGE OR THE OSTEOLOGY OF POLYODON FOLIUM. 
If the arrangement of the splints which invest the posterior cranial region of the 
Sturgeon throws light on the nature of those similarly located in Polyodon, it is also 
true that the disposition of the rostral splints in the latter Ganoid suggests the plan 
on which those of the former are arranged. 
In no two specimens of Acipenser are the anterior cranial splints disposed in quite 
the same way, and the arrangement may even differ on opposite sides of the same 
skull. From an examination of several skulls, and especially of one now in the 
University Museum, and from which the annexed woodcut was taken, I think it may 
be demonstrated that the splints in both genera are arranged in accordance with a 
common plan. 
Thus in the woodcut the splints marked a 1 to a 4 correspond to the medio-dorsal 
series in the typical scheme, and in Polyodon. The one marked a 1 is a dagger-shaped 
bone, and evidently represents the dermo-supraoccipital. The next mesial splint (o' 2 ) 
is a dermo-ethmoid, and in this specimen is separated from the preceding bone by the 
mesial apposition of b l b l — two bones belonging to the supero-lateral series — though in 
most skulls the long blade of the dermo-supraoccipital completely separates them, 
and suturally unites with the hinder margin of the ethmoid. 
The splints a 2 to a 4, continue this series to the end of the rostrum, and, as in 
Polyodon, are more or less completely separated from one another by the interpolation 
of the paired elements of the next series. The scute marked a is the first of the 
