722 
ME. T. W. BE1DGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
most distinctive feature about Acipenser and Polyodon is the remarkably Selachian 
character of their mandibular arches, and it is to those structures that we must look 
for definitions of the subsidiary groups. In both genera the pterygoid outgrowth 
from the proximal segment of each mandibular arch is united with its fellow in a 
median symphysis, as in the Selachii ; in all the remaining recent Ganoids these out- 
growths are connected through the intervention of a palatine bone with the prefrontal 
region of the skull. 
W e may therefore classify Ganoids into two main groups as follows : — 
Ganoidei, 
a. Selcichoidei . — Pterygoid processes united in a median symphysis. Persis- 
tent and unsegmented notochord. Persistent spiracles and mandi- 
bular branchiae. 
Genera — Polyodon, Acipenser, Scaphirhynchus, and Chondrosteus. 
ft, Teleosteoidei. — Pterygoid processes not united with each other, but 
connected directly, or indirectly through the intervention of a 
palatine bone, with the prefrontal region of the cranium. Vertebral 
column generally ossified into distinct vertebrae ; notochord aborted ; 
no mandibular branchiae. 
Genera — Amici , Polypterus, Calamoichthys, and Lepiclosteus. 
In deciding upon the claims of the fossil Ganoids to be included in one or the other 
of these two groups, we are compelled to rely altogether on the structure of the 
upper jaw. Tested in this way, we must refer the Crossopterygidae, Lepidosteidae, 
Palaeoniscidae, and Platysomidae to the group of Teleosteoid Ganoids. The Acan- 
thodidae probably belong to the Selachoid group. 
The distinctness of the Selachoid and Teleosteoid groups is materially lessened when 
an attempt is made to include the fossil genera of Ganoids in either of them. Acipenser 
and Chondrosteus in the former group, and the Palaeoniscidae and the Platysomidae in 
the latter, partially bridge over the gap which exists between the two when only the 
recent forms are considered. 
Dr. Traquair,'" in his valuable paper “On the Ganoids of the Carboniferous 
Formations,” refers the families Palaeoniscidae and Platysomidae to the suborder 
Acipenseroidei, which also includes Acipenser, Chondrosteus, and Polyodon. 
The principal characters by which this suborder is defined and distinguished from 
the other suborders of Crossopterygii, Amioidei, and Lepidosteoidei, as given by 
Dr. Traquair, are as follows ; — 
“ Suborder II. Acipenseroidei. — Tail completely heterocercal ; notochord persistent; 
paired fins non-lobate, infra- claviculars present, rays of dorsal and anal fins exceeding 
in number their supporting interspinous bones ; praeoperculum when present tending 
to extend forwards over the cheek ; branchiostegal rays in most, but large jugulars in 
* Op. cit. 
