686 MR. T. W. BRIDGE OR THE OSTEOLOGY OF POLYODOR FOLIUM. 
The marginal ossicles, d l cl 1 , partially overlap the preceding splints. Their rays 
unite with those of the star-shaped osseous spiculae by which the flexible lateral edges 
of the rostrum are strengthened and supported. 
Circum- orbital Bones. — The orbit is bounded behind, below, and in front by a 
series of four or five small elongated orbital bones. Of these, two are postorbital, 
and the others suborbital and preorbital elements. They very much resemble the 
corresponding bones in many Siluroids (Ex. Silurus, Synodontis, &c.). The last sub- 
orbital projects backwards beyond the postorbital series as in these genera. There 
are no supraorbital bones. The uppermost postorbital and the preorbital are loosely 
attached to the sphenotic process and to the inferior margin of the nasal opening 
respectively. 
The difficulty of correlating these investing parostoses with those which exist in 
other Yertebrata has been referred to by Professor Huxley (‘ Vertebrate Skull,’ p. 203), 
and is mainly due to the fact that ossification has continuously invaded the fibrous 
tracts, instead of originating in distinct and definite centres. Hence in Polyodon one 
continuous splint may cover regions of the subjacent chondrocranium, which in other 
Ganoids and in Teleostei are invested by several distinct splints. 
This difficulty is further enhanced by the want of a perfectly satisfactory scheme for 
the systematic classification and definite nomenclature of such superficial splints. The 
only attempt in this direction with which I am acquainted is that suggested by Professor 
Parker in his ‘Morphology of the Skull’ (pp. 343-346). From the consideration of 
the dermal armature in the Siluroid fish Callichthys, he regards its cranial splints as 
being the serial homologues of the supero -lateral and infero-lateral plates with which 
its body is invested. 
Thus the two supraoccipital derm bones, the single parietal, the paired frontals, 
and the azygous ethmoid are the serial homologues, whether azygous or paired, of the 
upper three-fourths of the superoffiateral body plates ; while the post- temporal, supra- 
temporal, dermo-sphenotic, the circumorbital ossicles, the lachrymals and nasals cor- 
respond to the ventral fourths of those plates, which have been segmented off, and serve 
for the transmission of the cranial prolongation of the lateral line mucus canal. The 
subdivision of the infero-lateral body plates gives rise in the skull to the opercular 
bones, maxillae, jugals, preemaxillae, branch iostegal rays, and the external mandibular 
splints. This scheme seems to me to be susceptible of one or two slight modifications, 
which were suggested to me by a careful examination of the very generalised splints 
that invest the posterior cranial region of the Sturgeon and the rostrum of Polyodon. 
We may, I think, with advantage classify the body splints into four series — a median 
dorsal, a supero -lateral, a lateral, an infero-lateral, and a median ventral series, the 
lateral series being devoted to the transmission of the lateral slime canal, and each 
element perforated by a mucous pore. This arrangement is practically that which 
obtains in the post-cranial armature of the Siluroid genus Loricaria. The median 
dorsal series is represented in the trunk by the median row of scales or scutes, which 
