706 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
there is a prepalatine spur in front of the short ethmo- palatine connexion with the 
prefrontal and a postpalatine region behind which abuts against the mesopterygoid 
and pterygoid elements of the pterygoid outgrowth. It may be mentioned that the 
recorded facts of the development of the Teleostean skull show that the palatine is 
at first quite distinct both from the ethmo-trabecular and quadrate regions, as seen 
in Plates 1-3 of Mr. Parker’s ‘ Memoir on the Structure and Development of the 
Salmon’s Skull/'"' Subsequently the palatine fuses behind either with the quadrate 
or with its pterygoid outgrowth. 
In the Elasmobranchs the same structures are readily recognized. In some Selachii 
(Notidanus) and in some Batoidei {Raja and Rhinchobatus ) there is a large recurrent 
cartilaginous process attached to the hinder wall of each nasal capsule, but in many 
Selachians, as in Cestracion and in Mustelus, there is no trace of any such free process. 
In Raja the thick proximal end of this process is perforated by a foramen for the 
transmission of the superior maxillary division of the fifth nerve, as is the case with 
the palatine process of the Conger Eel ; and in Rhinchobatus it presents a division 
into regions strictly comparable to the prepalatine, ethmo-palatine, and postpalatine 
regions of the palatine process of the Teleostean and the Amphibian. In all Elasmo- 
branchs a forward growth from the proximal half of the mandibular arch unites 
beneath the prefrontal region with its fellow of the opposite side to form the charac- 
teristic upper jaw of this group. 
That this retral antorbital process of the Sharks and Rays is homologous with the 
palatine process of the Toad, the Axolotl, and the Siluroid, seems to me an incon- 
testable fact, proved by its attachment to the prefrontal region, by its perforation in 
Raja, and by its regional subdivision in Rhinchobatus. But if this retral outgrowth 
be a true palatine element, then the forward outgrowth from the mapibular arch — 
the so-called palato-pterygoid bar — must correspond to the pterygoid process of the 
Toad and Axolotl, and cannot therefore contain any palatine element. Consequently 
the term “ palato- quadrate arcade” is not rightly applicable to the confluent pterygoid 
and quadrate cartilages, and ought to be replaced by the term “ pterygo-quadrate.” 
We are now in a better position to determine the nature of the upper jaw and 
its ossifications in Polyodon. It is evident that in so far as relates to the palatine 
and pterygoid processes, this Ganoid so closely resembles those Selachii in which the 
mesially-united pterygoids are unaccompanied by even the rudiments of recurrent 
palatine outgrowths, that we must conclude that its upper jaws are formed in precisely 
the same way, i.e., by the union of the pterygoid bars. The two ectosteal patches 
without doubt, therefore, correspond to the mesopterygoid elements of other Vertebrata. 
It is impossible to say whether the palatine process has entirely aborted, or whether 
it has contributed to form any part of the thickened and somewhat produced posterior 
wall of the nasal capsule. 
An alternative, suggested by the slightly upturned anterior end of each half of the 
* Phil. Trans., 1872. 
