MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
G91 
medio-dorsal plates of the trunk. In the supero -lateral series, the paired osseous 
plates marked b l and 6 3 are the parietals and frontals ; and, as in Polyodon, the former 
are in contact mesially, though partially separated behind by the dermo-supraoccipital, 
which, as a distinct bone, does not exist in the latter Fish. 
The bones 6 3 and b 4 ‘ repeat the parietals and frontals along the sides of the snout. 
The splint c 1 (post-temporal), the first splint of the third, or infero-lateral series, 
resembles the corresponding bone in Polyodon in developing an extensive descending 
osseous lamina which is applied by its anterior margin to the cartilage that forms the 
lateral walls of the cranio-spinal canal. The splints c 3 -c 5 , which are represented by a 
single bone (c 3 ) in Polyodon, correspond to the supra-temporal, dermo-sphenotic, supra- 
orbital, and ecto-ethmoid elements of the Teleostean skull, c 0 is a nasal bone, and 
c 7 and c 8 continue this series forwards along the rostrum. 
Postorbital, suborbital, and prseorbital bones surround the orbit, and the inferior 
margin of the rostrum is fringed by a series of five or six plate-like marginal ossicles 
( m.o .) precisely as in Polyodon. 
Thus it will be seen that, notwithstanding the superficial differences between the 
cranial splints of the two genera, due to the tendency in Polyodon of the osseous 
centres to continuously invade fibrous tracts usually ossified separately, and to the 
irregular asymmetrical disposition and proneness to segmentation of the splints in 
Acipenser, their fundamental arrangement is much the same in both Ganoids. 
The foregoing considerations seem to me to suggest an explanation of the dermal 
armature of Ceratodus, which is somewhat different from that proposed by Professor 
Huxley. * Comparing the skull of that Fish with those of Acipenser and Polyodon, 
it will be seen that the anterior and posterior median bones (A 1 and A 3 ) in the former 
are the equivalents of two elements in the medio-dorsal series of the latter, viz., the 
dermo-supraoccipital and dermo-ethmoid, while the anterior and posterior inner lateral 
bones (C 1 and D 3 ) correspond to the parietals and frontals. f 
The bones marked E and D are elements of the lateral series, and probably 
correspond to the bones called supra-temporal in the Sturgeon, or, if coalescent, they 
would be the equivalent of the greater part of the bone c 3 in Polyodon. 
But as Dr. Gunther remarks the fact that all the cranial derm bones in Ceratodus, 
with the exception of the median ethmoid, lie external to the temporal muscles, throws 
considerable doubt on the results of any attempt to identify these bones with those 
which in other Vertebrata lie beneath the muscles and directly upon the cartilage of 
the cranial roof. The former are evidently ossifications of a more superficial fibrous 
tract than that in which the latter are usually formed. 
If the two splints which invest the rostrum of the Pike’s cranium, and are marked 2-2 
* “ Contributions to Morphology. Ichthyopsida, No. 1. — On Ceratodus fosteri, with Observations on 
the Classification of Fishes.” Proc. Zool. Soc., 1876. 
t It will be noticed that in Ceratodus, as is commonly the case in Acipenser, the dermo-ethmoid and 
dermo-supraoccipital bones are suturally united. 
