MOHPHOLOGY OF TELKOSTKAN HEAD SKELETON. 521 



Of tlie otic bones (figs 9, 12, o. e.,o. 'p., o. sj),, o. pr.) the 

 opistliotic is absent, and the remainder are represented by 

 rounded areas of ossification, but are still widely separated 

 by cartilage. The circular form and wide separation of the 

 ossifications calls to mind the condition of the same set of 

 bones in the adult Amia (Sagemehl, 84, p. 202). The main 

 part of the pro-otic (o. pr.) completely surrounds the exit for 

 the seventh nerve, whilst its anterior border is deeply 

 indented for the passage of the second and third branches of 

 the fifth. The portion which lies between these two nerves 

 is the portion hitherto called the pro-otic process. The front 

 part of the sphenotic is formed by ossification of the plate-like 

 extension of the auditory capsule — the sphenoidal region. 



In the younger individuals of this stage, except for the 

 pro-otic, each of these ossifications is confined mainly to the 

 external surface of the cartilage. Under the central portions 

 of the epiotic, pterotic, and sphenotic the cartilage has 

 disappeared, and consequently this outer lamina of bone 

 comes to form part of the internal wall of the capsular and 

 cranial cavities. Apart from this, the condition exhibited at 

 this stage bears a close resemblance to that found in Amia 

 (Bridge, 77, p. 612), in which the pro-otic is the only bone of 

 this series visible internally. 



In the anterior portions of the skull the expanded plate-like 

 portions of the parethmoid cornua have given rise to the 

 parethmoid bones (figs. 4, 9, e. p. ?>.), whilst a centre of ossifi- 

 cation, the mesethmoid {e. m.), has appeared on the dorsal 

 surface of the corresponding cartilage. The edges of the 

 latter ossification extend freely into the surrounding tissue, 

 and give the impression of a membrane bone whose 

 central portion has united with the cartilage, leaving the 

 edges quite free. Though no further evidence that this has 

 actually taken place is forthcoming in any of the examples 

 which I have examined, there can be little doubt that the 

 impression thus formed is a true one. In the salmon 

 (Parker, 73, pp. 98, 138) this bone exists even in the adult 

 as a membrane bone, perfectly free of the underlying 



