[Proc. Roy. Soc. Victoria, 35 (N.S.), Pt. II., 1923]. 



Art. X. — The Occipital Bones of the Dipnoi, 



By HARLEY S. BAIRD, B.Sc. 



(Zoological Laboratory, Melbourne University). 



(Communicated by Professor W. E. Agar.) 



[Read lOtb August, 1922.] 



In 1897 Bridge made the statement, " The only endochondral bone» 

 in Ceratodus are the two exoccipitals." K. Furbringer (1904) draws 

 attention to the fact that Bridge separates these bones sharply from 

 all other bones in the Ceratodus skull on the grounds that they are 

 endochondral. Goodrich (1909) accepts Bridge's view of the nature 

 of the bone in the words, " The bone described by Huxley as an. 

 exoccipital, the only endochondral ossification in the skull of any 

 Dipnoi, appears to represent the first of the occipital neural arches." 



That the exoccipital bone of the Dipnoi is the enlarged first neural 

 arch has been shown, by K. Furbringer (1904) for Ceratodus^ and by 

 Agar (1906) for Lepidosiren and Protopterus. The reasons for con- 

 sidering it to be an endochondral bone do not appear ever to have 

 been definitely stated. As the question is of some importance in 

 deciding the evolutionary status of the Dipnoi, a histological exami- 

 nation of the bone was undertaken at Professor Agar's suggestion. 

 Well preserved Ceratodus skulls were kindly provided by Dr. T. L. 

 Bancroft, of Eidsvold, Queensland, and also by the Director of the 

 Queensland Museum. Comparative material was available in the form 

 of larvse and young of Lepidosiren. 



Much depends upon the definition of the term endochondral, which 

 appears to be used in different senses. Adopting Goodrich's classifica- 

 tion of the types of bone found in Fishes (1909, p. 63), and which 

 corresponds with Gaupp's nomenclature (1906, p. 610), cartilage bone 

 is of two kinds, perichondral and endochondral. In the former, the 

 bone is deposited layer upon layer by osteoblasts lining the inner 

 surface of the perichondrium (which now becomes the periosteum), 

 thus gradually restricting or replacing the cartilage from outside 

 inwards. Endochondral bone, on the other hand, is formed, not in 

 this, but rather in the opposite way. Spaces are eaten out of the 

 cartilage by osteoclasts, and bone is deposited in the interior of the 

 cavities so formed, and spreads outwards. 



Examination of sections of the bone in half-grown Ceratodus 

 skulls, and of the development of the bone in Lepidosiren, gives ne 

 evidence of the formation of endochondral bone in the above sense. 



In Ceratodus the exoccipital bones are imbedded in the side walls 

 of the chondrocranium. Each is a small hollow bone shaped rather 



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