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MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
I. The very generalised cranial splints arranged in a medio-dorsal, a snpero -lateral, 
and a lateral series, investing the dorsal surface of the chondrocranium, and cor- 
responding to the dermo-ethmoid, the parietal and frontals, the post-temporal, the 
dermo-sphenotic, and the nasal bones of other Ganoids and Teleostei ; the serial repe- 
tition of these dermal elements along the dorsal surface of a massive and prolonged 
prenasal rostrum ; the median parasphenoid and paired vomers, which in like manner 
are repeated by a series of homologues applied along the ventral surface of the 
rostrum ; while the infero-lateral series of splints includes an operculum, an inter- 
operculum, a maxilla, and a multi-segmented premaxilla, but no preoperculum, sub- 
operculum, branchiostegal rays, or median or paired jugular plates. 
II. The massive and persistent chondrocranium entirely devoid of any such 
endosteal ossifications as basioccipital, exoccipital, alisphenoid, orbitosphenoicl, or pre- 
frontal elements, all of which are represented in the crania of the remaining recent 
Ganoidei, if we regard Ceratodus and Lepidosiren as being members of a distinct 
primary group, the Dipnoi ; while diminutive scale-like opisthotic and pterotic exos- 
toses alone represent in Polyodon the well-ossified periotic bones of other Ganoids and 
Teleostei. 
III. The union of the anterior ends of the persistently cartilaginous quadrato-ptery- 
goid arcades of their hyostylic skulls in a medio-ventral symphysis beneath the 
olfactory capsules, as in the Selachii. Associated with each arcade, which is suspended 
from the hyomandibular by a pair of symplectic ligaments, there are ectosteal meso- 
pterygoid plates, and a parosteal pterygoid bone ; and in addition, in Acipenser, a well- 
ossified palatine element. The proximal segment of the mandibular arch is either 
represented by a metapterygoid cartilage ( Acipenser ), or by a strong metapterygoid 
ligament passing from the skull downwards and backwards beneath the spiracle to its 
insertion into the upper third of the hyomandibular ( Polyodon ). 
IV. The enormous backward extension of the gape, due to the great posterior 
inclination of the hyomandibular and quadrate ; and the production of the latter into 
a large and persistent “ orbitar process,” between which and the quadrato-pterygoid 
bar the levator mandibularis muscle lies, precisely as is the case with the Tadpoles of 
the Anurous Amphibia. 
Y. The presence of persistent spiracles, and of mandibular branchiae attached to the 
posterior margin of the metapterygoid ligament [Polyodon). 
YI. The perforation of the outer wall of the periotic capsule by an antero-posterior 
canal, as in the Elasmobranchs Trygon, Pristis, and Phinchobatns ; and the fenestra- 
tion of the roof of the capsule by a canal passing beneath the arch of the posterior 
vertical semicircular canal to communicate with the cranial cavity, which may possibly 
represent the orifice of the primitive involution to form the internal ear. 
VII. The persistent unossified Meckelian cartilages, flanked by dentary and splenial 
elements, and with ossified lower labial cartilages (mento-Meckelian bones) interposed 
between their symphysial ends. 
