336 



TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



for the symplectic. In the central parts of these tracts, which are subject to great stresses, 

 the trabeculae are so crowded together that the bone Is more or less opaque even to strong 

 light; but these dense parts are connected by a web of bone of varying thickness, with zonal 

 growth-bands traversed by trabecular ridges that radiate from the growth-center of the 

 bone. In the quadrate the growth-center appears to be located in the dense articular facet 

 at the lower end; from this the bone has seemingly grown upward in a fan-like way, a very 

 strong ridge near the hinder border carrying the growth lines obliquely upward while the 

 web-like plate of the bone has grown by transverse zones crossed by upwardly-streaming 



rays. 



The endochondral bones of the braincase (Fig. 215^) likewise consist of zonal and 

 trabecular regions. On the side of the braincase there is a prominent triradiate suture 

 marking the adjacent boundaries of the pterotic, prootic and exoccipital bones; each of these 

 exhibits zonal plates strengthened by trabecular ridges. The triradiate suture is probably 

 somehow due to uniform rates of growth from three equally distant centers. Another 

 triradiate suture separates the prootic, the alisphenoid and the sphenotic. 



60 3 



g^l^r so^ SOS pto pa 



Aspicottus 



Fig. 216. Aspicottus (Enophrys), with oblique view of 80». 



In a general way the ectosteal or derm bones resemble the endochondral bones in being 

 composed of zonal and trabecular elements. The same principles of growth are clearly 



