712 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
that the young Polyodon possessed a suctorial mouth and passed through larval 
stages in several respects very similar to those of the former. 
The study of the cranial development of Acipenser and Polyodon will probably 
reveal new and important anatomical features which are common to the Ganoids and 
the Amphibia. 
Shoulder Girdle. 
Dr. Traquair, in his memoir “ On the Palseoniscidse ” (plate vii., fig. 1), supplies a 
sketch of the membrane bones which invest the cartilaginous pectoral girdle.'" As to 
the identity of the bones lettered l.cl. and c. 1. there can be no doubt ; they correspond 
to the clavicle and inter-clavicle of other Ganoids. The bone marked p.cl. is clearly 
the equivalent of the bone which in Acipenser is called supra-clavicle by Parker ; but 
I agree with Dr. Traquair in regarding it as a post-clavicle, the true supra-clavicle 
being the long and sabre-slraped bone marked s.cl. The supra-clavicle is attached to 
the postero-external angle of the bone c l , the squamosal of Traquair, which evidently 
corresponds to the bone which in the Sturgeon Parker calls the supra-temporal. Bat 
this splint (c 1 ) corresponds in every respect to the post-temporal bone of other Ganoids; 
its horizontal and descending lamina are analogous to the two processes into which 
the Piscine post-temporal ordinarily divides, and like the latter serve to attach the 
pectoral girdle to the postero-lateral angles of the skull. 
The cartilaginous part of the pectoral arch, though far less massive and of more 
delicate proportions, is essentially the same as in the Sturgeon ; but while the latter 
resembles the Skate in having a distinct supra-scapular segment, Polyodon agrees with 
the Selachii in having that element confluent with the scapular. 
It may be remarked that in both of these genera the scapular arch is essentially the 
same as in Elasmobranchs, with the exception that the large and distinct prsecoracoid 
of the former has no representative in the latter. An examination of any ordinary 
Shark — Acanthia or Squatina for example — will show that the scapula segment is 
divided by a scapular fenestra into a scapula proper and a pre-scapular element, and, 
further, that there is a coraco -scapular fenestra perforating the coracoid cartilage 
immediately ventrad of the glenoid facets and opening into the floor of the scapular 
fenestra precisely as in Polyodon and Acipenser. Such a coraco -scapular fenestra is 
found in nearly all the Selachii which I have had the opportunity of examining. 
The epicoracoid process in Polyodon does not extend so far along the inner surface 
of the inter-clavicle as in Acipenser. 
The coraco-scapular cartilage is much the same in Amia, though much reduced in 
size proportionally to the size of the overlying derm bones; and the same may be 
said of these structures in Lepidosteus, though in the latter, scapula and coracoid 
are largely ossified. In both genera there is the same longitudinal clearage of the 
cartilage into distinct morphological elements by coraco-scapular and scapular fenestrse, 
having the same relations to each other as in Polyodon. 
* For figures of coraco-scapular arch, vide Gegenbaur, op. cit., Part I., plate vi., figs. 3a, 3b, and 3c. 
