116 Harley S. Baird .- The Occipital Bones of the Dipnoi. 



like an hour glass. The anterior end is slightly more expanded than 

 the posterior, and the whole bone is filled with cartilage (even in the 

 adult), with the exception of the narrow constricted region, where it 

 is bone all through. Transverse sections of one of the exoccipitals 

 from a young Ceratodus showed that the ossification was taking place 

 in the periosteum, i.e., it was perichondral. No traces of endochondral 

 bone formation could be observed at all, and since the ossification 

 was very nearly as complete as it ever would be, it seems unlikely 

 that it would occur in a later stage. 



The general shape of the occipital bones in Leindosirem is very 

 similar to that of Ceratodus, but proportionally they are much larger, 

 as they themselves form the lateral walls for the posterior region of 

 the skull, their anterior surfaces only being in contact with the 

 cartilage of the auditory capsule. They are slightly constricted In 

 the middle owing to their being grooved anteriorly by the vagus, and 

 posteriorly by the first spinal nerve. In the adult Lepidosiren, the 

 €xoccipital " bones " are very nearly completely ossified, only a small 

 plug of cartilage remains at the dorsal end. Examination of transverse 

 sections of the occipital region at different stages of development 

 indicate that the formation of bone begins in the periosteum, and 

 gradually spreads inwards, upwards, and downwards, replacing the 

 cartilage. As in Ceratodus no definite signs of endochondral ossifica- 

 tion could be found at any stage. 



It would appear therefore that there is not sufficient foundation 

 for the statement that the exoccipital bone in Ceratodus or other 

 Dipnoi is an endochondral bone. Such mode of ossification does no-t 

 seem to have evolved yet in the Dipnoi. 



REFERENCES. 



Agar, W. E.— Trans. Roy. Soc. Edinburgh, 45, 1906. 



Bridge, T. W.— Trans. Zool. Soc. London, 14, 1898. 



Gaupp, B. — In Hertwig's Handbuch der Vergleichenden und experimen- 

 tellen Entwicklungslehre der Wirbelthiere, 1906. 



Goodrich, E. S. — Lankester's Treatise on Zoc'logy, 1909. 



JPiirbringer, K. — In Semon's Zoologische Forschungsreisen in Aus- 

 tralian, 1904. 



