704 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
To determine the homologies of the two ectosteal patches ( m.pg .) previously described 
as developed in connexion with the palato-pterygoid bar, is a task which is not very 
easy, and in order to do so it is necessary to form a true conception of the nature of 
the “palato-pterygoid” bar as it exists in different Vertebrata. This bar has been 
generally regarded as a forward outgrowth from the mandibular arch, which is even- 
tually ossified from several distinct centres and so gives rise to the palatine, meso- 
pterygoid, and pterygoid bones, the first mentioned being an ossification in the most 
anterior moiety of the bar. If this view be correct our task is an easy one, and we 
may at once regard the bone m.pg. as being the equivalent of the palatine bone of 
other Fishes. 
But Professor Parker has s unnested another view as to the nature of this bar 
oo 
in the Vertebrata generally. In his opinion it is formed by the coalescence of a retral 
growth from the ethmoidal region (palatine process) with. a forward outgrowth from 
the mandibular arch (pterygoid process). In accordance with this view the palatine 
bone is an ossification in the retral process, and is a preoral structure, while the meso- 
pterygoid and pterygoid are developed in relation with the cartilaginous pterygoid 
process, and are therefore postoral elements. If, therefore, the upper jaw in Polyodon 
is formed by the union of the pterygoid processes only, then the ectosteal patches 
represent a mesopterygoid element. The evidence in favour of this theory as to the 
nature of the palato-pterygoid bar is very strong, and may be briefly set forth. 
In the common Toad, in the adult skull, the primitive distinctness of the palatine 
and pterygoid processes is well seen. The cartilaginous pterygoid process forms less 
than half of the curvature of the subocular bar, the anterior part of which is formed by 
the retral palatine process. From the inner side of this process a transverse piece of 
cartilage, the etlnno -palatine, is given off, and is connected by fibrous tissue with the 
prefrontal region (ethmo-trabecular outgrowth). In front of the ethmo-p alatine the 
palatine process is produced into a prepalatine spur, and behind it into a postpalatine 
process which is in ligamentous connexion with the anterior end of the pterygoid 
process.'" In the Frog the segmentation of the palatine and pterygoid outgrowths 
has not been observed, neither has the primitive formation of this bar by the coales- 
cence of retral and forward outgrowths been observed 'in any embryo Anura. This 
important desideratum is however supplied by the developmental history of the embryo 
Axolotl. In young embryos of 1^, 2^, and 3^ inches long, the stages A, B, and C 
of Parker, the formation of these processes has been observed (‘ Morphology of the 
Skull,’ Parker and Bettany, pp. 109-112). In stage A there is not the slightest 
trace of a pterygoid process in connexion with the suspensorial cartilage, but in the 
antorbital region a small transversely disposed rod of cartilage has appeared which is 
attached by ligament to the trabecular cartilage just above the broad tooth-bearing 
part of the palatine splint. This transverse rod is the homologue of the palatine 
process of the Toad. In stage B a pterygoid process has appeared growing forwards 
* Parker, Phil. Trans., 1876, Plate 54, fig. 3. 
