406 H- B. POLLARD, 



preorbital caoal was only slightly ossified at its commencement and 

 the groove for the ophthalmicus superficialis and i)r()fundus was scar- 

 cely ossified at all. More ventrally towards the ethmopalatine articu- 

 lation the ectethmoid ossification was almost complete scarcely any 

 trace of cartilage being left. In an older specimen of which the nasal 

 region was cut in sections the groove was partially ossified, the ecteth- 

 moid supporting the nasal bones (figs. 9, 11). 



Again the foramen for the exit of the Trigeminus (fig. 28) lay 

 behind the orbitosphenoid which only formed part of its anterior and 

 inferior boundary. The rest was formed purely by cartilage. 



Here then in two critical instances we see that the bone does not 

 arise to protect the exit of the nerve. Some other reason must there- 

 fore exist and that view seems more probable which suggests that 

 bones arise through Natural Selection, at the periphery of cartilage 

 where they are of actual use to meet mechanical stress from the 

 action of muscles or the working of joints. 



The ectethmoid arises at the ethmopalatine articulation, secondarily 

 also supporting the frontal at its anterior extremity (fig. 'do). In the 

 palatine cartilaginous bar at the same time an autopalatine is formed 

 just at the articulation and not extending either forward to its tip or 

 backwards. 



The orbitosphenoid extends from the parasphenoid to the frontal 

 sustaining the stress of the temporal muscle which is largely affixed 

 to the free edge of the frontal. A simple model of its action may 

 be made by hanging a string with a heavy weight at each end over 

 two boards placed with their bases together and leaning apart. Po- 

 steriorly the frontal also is supported by the spheuotic and postfrontal, 

 the sphenotic in turn rests on the wing of the parasphenoid. The opi- 

 sthotic besides giving attachment to muscles and ligaments, supports 

 especially the posttemporal scale, which will be referred to again 

 later. Below it rests on the exoccipital and wing of the parasphenoid. 

 In old specimens the squamosoparietal process fuses with it. 



The exoccipitals with the fused vertebrae articulate with the verte- 

 bral column and support especially the opisthotic against the stress 

 of the neck muscles. 



Other good examples are seen in the pterotic of Teleostei and in 

 the vertebral column, where vertebrae primitively arise to support the 

 ribs and processes. The bones thus picked out, to use a figurative ex- 

 pression, may attain a greater or less independence. 



The first suprapharyngobranchial articulates with 



