MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
689 
horizontal and descending plates would appear to correspond to the two processes for 
attachment to the postero-lateral angles of the skull into which the post-temporal bone 
is divided in Amia, Lepidosteus, and in the majority of Teleostei. 
In the Sturgeon, however, Professor Parker* has described a complete series ot 
pectoral splints, consisting of an interclavicle, a clavicle, a supraclavicle, and a post- 
temporal, the latter being attached to the skull by the bone marked c 1 (fig. 1, p. 690), 
(supra-temporal, Parker), which, in possessing horizontal and descending laminae, is 
in every way comparable to the bone designated c 1 in Polyodon (Plate 55, fig. 1). That 
these two bones are homologues I have but little doubt, and if the bone called post- 
temporal in Acipenser is correctly so named, then we must regard both the former as 
being supra-temporal elements, the proper post-temporal being altogether suppressed 
in Polyodon . 
Dr. Traquair,! in referring to the shoulder-girdle of Polyodon, calls the bone c 1 
the squamosal, but as this term is now usually applied to the bone which in the 
higher Yertebrata results from the coalescence of the prseoperculum with a supra- 
temporal element, it would seem better, if it represents any part of the compound 
squamosal, to regard c l as being a supra-temporal element only. I am, however, 
much inclined (for reasons to be mentioned in describing the shoulder-girdle) to regard 
the splint in question as being, in both Ganoids, the homologue of the post -temporal 
of Teleostei. 
The splint c 3 overlies the pterotic ridge and the sphenotic^ process, forms a roof to 
the orbit, and extends forwards far enough to overlie the nasal capsules. I imagine 
that this splint represents the distinct supra-temporal, dermo-sphenotic, and, perhaps, 
the ecto- ethmoidal elements of Acipenser. The thickened sphenotic portion of this 
bone suggests that ossification first began in that region, but afterwards continuously 
invaded fibrous tracts in front and behind which in the Sturgeon are separately 
ossified from distinct centres. , 
The parostosis c 3 is apparently a nasal bone, but may possibly be a dermal ecto - 
ethmoid, and the splints which I have described as being applied to the supero -lateral 
margin of the cartilaginous rostrum, serially repeat these elements of the lateral line 
series. 
The rayed marginal ossicles belong to the infero-lateral series, and the first of them 
is applied to the outer side of the nasal bone (c 3 ). It is hardly possible to correlate 
these ossicles with any that exist in other Fishes, but perhaps they may be regarded as 
corresponding to that part of the infero-lateral series which is generally represented 
in other Fishes by the premaxillse. In Acipenser they are replaced by a series of bony 
plates which bound the infero-lateral margin of the rostrum. 
* “ Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the 
Vertebrata.” Ray Society, 1878. 
t “ Ganoid Fishes of the British Carboniferous Formations, Part I. — Pahnoni, seiche.” Mem. Palasonto- 
graphical Soc., 1877. 
4 T 2 
