On the Anatomy and Phylogenetic Position of Polypterus. 405 



the similar phenomenon in Polypterus. There is some resemblance in 

 the nasal capsule. 



The rest of the Urodeles depart less from Polypterus in the 

 relations of the antorbital region of the nasal capsule. In the Axolotl 

 some approach also is made in the form of the rostral cartilage though 

 in a different way from Siren. 



Other resemblances no doubt might be found on careful search 

 for which at present I have not the requisite knowledge. 



Traquair and Bridge have pointed out the similarity of the 

 orbitosphenoid in its later stages to the sphenethmoid of the Anura. 

 A likeness is also seen in a median and longitudinal section of the 

 skull. The Anura are well known to ditier very much anatomically from 

 the Urodela and it is by no means astonishing that certain distinctive 

 characters of the Anura can be traced in Polypterus. In the Anura 

 the tegmen cranii reappears. 



The Amphibia diScr from the Crossopterygian form in: 



(1) the non-inclusion of vertebral elements (that is of the two 

 ossified vertebrae of Polypterus) in the skull, 



(2) the absence of an ossified ectethmoid corresponding with 

 the loss of an ethmopalatinc articulation, 



(3) in the presence of true posterior nares, 



(4) in the appearence of a prootic with the simultaneous re- 

 duction of the parasphenoid. 



Thus it has been indicated that the p rimor dial cranium of 

 Polypterus admits of comparison with the skull of 

 Chlamydoselachus showing also similarities witht he Notidanklae. 

 It also leads on to the Amphibia which however acquire 

 several new and distinctive characteristics. 



On the commencement of bone formation. 



No animal, accessible for study, is known to me which can be 

 considered to lie between the Selachii and Polypterus or in other 

 words in the latter we have the earliest form with an ossified primor- 

 dial cranium and it therefore will be the most suitable type to explain 

 the origin of perichondria! bones. In others "abbreviation of deve- 

 lopment" becomes a factor. It is usually stated that such bones arise 

 at the exits of important nerves for the protection of the same. 



In my youngest specimen which I cut into sections the ossification 

 though considerable had by no means reached the adult stage. The 



